TST : Added more tests to test_recip

Uses test that has one point in the center of each grid.
Now checks that all points are in grid, the occupancy is correct, and
that intensity array is exactly equal.
TST : added second test to test error

Added second test of gridder
ENH : made exceptions have useful messages

Now tell the user what the ValueError is (shape of input data)
DOC : corrected documentation of gridder

the error value is actually the standard error, not the standard
deviation.

Made other docs more verbose