Issue:
PBC DFT Becke grid weights wrong

PySCF version: [fd8ee3e](https://github.com/pyscf/pyscf/commit/fd8ee3e264affb668874c543562a46239c66871e)
Input:
```
import numpy as np
from pyscf.pbc import gto, dft

atom = '''
Cu1    1.927800000000   1.927800000000   1.590250000000
Cu1    5.783400000000   5.783400000000   1.590250000000
Cu2    1.927800000000   5.783400000000   1.590250000000
Cu2    5.783400000000   1.927800000000   1.590250000000
O1     1.927800000000   3.855600000000   1.590250000000
O1     3.855600000000   5.783400000000   1.590250000000
O1     5.783400000000   3.855600000000   1.590250000000
O1     3.855600000000   1.927800000000   1.590250000000
O1     0.000000000000   1.927800000000   1.590250000000
O1     1.927800000000   7.711200000000   1.590250000000
O1     7.711200000000   5.783400000000   1.590250000000
O1     5.783400000000   0.000000000000   1.590250000000
Ca     0.000000000000   0.000000000000   0.000000000000
Ca     3.855600000000   3.855600000000   0.000000000000
Ca     7.711200000000   3.855600000000   0.000000000000
Ca     3.855600000000   7.711200000000   0.000000000000
'''

cell = gto.Cell()
cell.atom = atom
cell.a = np.array(
  [[7.7112,    0.,    0.],
   [0.,    7.7112,    0.],
   [0.,    0.,    3.1805]])
cell.verbose = 5
cell.build()

grids = dft.gen_grid.BeckeGrids(cell)
grids.build()
```
Output:
```
tot grids = 270432
cell vol = 1276.25053  sum(weights) = 1256.85607
```

If I comment out the following code
https://github.com/pyscf/pyscf/blob/fd8ee3e264affb668874c543562a46239c66871e/pyscf/pbc/dft/gen_grid.py#L186-L194
I can get the correct result:
```
tot grids = 270432
cell vol = 1276.25053  sum(weights) = 1276.46051
```
although I don't understand the reason of this.

Fix patch:
diff --git a/pyscf/pbc/dft/gen_grid.py b/pyscf/pbc/dft/gen_grid.py
index d5b316a1f6..1cc4d9fa91 100644
--- a/pyscf/pbc/dft/gen_grid.py
+++ b/pyscf/pbc/dft/gen_grid.py
@@ -165,6 +165,7 @@ def get_becke_grids(cell, atom_grid={}, radi_method=dft.radi.gauss_chebyshev,
     b = cell.reciprocal_vectors(norm_to=1)
     supatm_idx = []
     k = 0
+    tol = 1e-15
     for iL, L in enumerate(Ls):
         for ia in range(cell.natm):
             coords, vol = atom_grids_tab[cell.atom_symbol(ia)]
@@ -174,24 +175,24 @@ def get_becke_grids(cell, atom_grid={}, radi_method=dft.radi.gauss_chebyshev,
 
             mask = np.ones(c.shape[1], dtype=bool)
             if dimension >= 1:
-                mask &= (c[0]>=-.5) & (c[0]<=.5)
+                mask &= (c[0]>-.5-tol) & (c[0]<.5+tol)
             if dimension >= 2:
-                mask &= (c[1]>=-.5) & (c[1]<=.5)
+                mask &= (c[1]>-.5-tol) & (c[1]<.5+tol)
             if dimension == 3:
-                mask &= (c[2]>=-.5) & (c[2]<=.5)
+                mask &= (c[2]>-.5-tol) & (c[2]<.5+tol)
 
             vol = vol[mask]
             if vol.size > 8:
                 c = c[:,mask]
                 if dimension >= 1:
-                    vol[c[0]==-.5] *= .5
-                    vol[c[0]== .5] *= .5
+                    vol[abs(c[0]+.5) < tol] *= .5
+                    vol[abs(c[0]-.5) < tol] *= .5
                 if dimension >= 2:
-                    vol[c[1]==-.5] *= .5
-                    vol[c[1]== .5] *= .5
+                    vol[abs(c[1]+.5) < tol] *= .5
+                    vol[abs(c[1]-.5) < tol] *= .5
                 if dimension == 3:
-                    vol[c[2]==-.5] *= .5
-                    vol[c[2]== .5] *= .5
+                    vol[abs(c[2]+.5) < tol] *= .5
+                    vol[abs(c[2]-.5) < tol] *= .5
                 coords = coords[mask]
                 coords_all.append(coords)
                 weights_all.append(vol)


Test patch:
diff --git a/pyscf/pbc/dft/test/test_gen_grid.py b/pyscf/pbc/dft/test/test_gen_grid.py
index 525f5f044a..b8ef48d1b1 100644
--- a/pyscf/pbc/dft/test/test_gen_grid.py
+++ b/pyscf/pbc/dft/test/test_gen_grid.py
@@ -125,6 +125,32 @@ def test_becke_grids_2d_low_dim_ft_type(self):
         s1 = get_ovlp(cell, grids)
         self.assertAlmostEqual(numpy.linalg.norm(s1-s2), 0, 5)
 
+    def test_becke_grids_round_error(self):
+        cell = pgto.Cell()
+        cell.atom = '''
+            Cu1    1.927800000000   1.927800000000   1.590250000000
+            Cu1    5.783400000000   5.783400000000   1.590250000000
+            Cu2    1.927800000000   5.783400000000   1.590250000000
+            Cu2    5.783400000000   1.927800000000   1.590250000000
+            O1     1.927800000000   3.855600000000   1.590250000000
+            O1     3.855600000000   5.783400000000   1.590250000000
+            O1     5.783400000000   3.855600000000   1.590250000000
+            O1     3.855600000000   1.927800000000   1.590250000000
+            O1     0.000000000000   1.927800000000   1.590250000000
+            O1     1.927800000000   7.711200000000   1.590250000000
+            O1     7.711200000000   5.783400000000   1.590250000000
+            O1     5.783400000000   0.000000000000   1.590250000000
+        '''
+        cell.a = numpy.array(
+            [[7.7112,    0.,    0.],
+             [0.,    7.7112,    0.],
+             [0.,    0.,    3.1805]])
+        cell.build()
+
+        grids = gen_grid.BeckeGrids(cell)
+        grids.level = 1
+        grids.build()
+        assert abs(cell.vol - grids.weights.sum()) < .1
 
 if __name__ == '__main__':
     print("Full Tests for Becke grids")