Repository: hgrecco/pint

Issue:
Concentration Units
Hi, 

Is there any reason concentration units (in particular molarity, mol / L) is not included as a default unit in Pint? It is very easy to define on its own, but it seems well-used enough to warrant inclusion in the defaults list. If there are no objections to adding it, I'm happy to submit a PR doing so.

Discussion:

Patch:
diff --git a/.gitignore b/.gitignore
index e39c2e8..545148c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,6 @@ MANIFEST
 
 # WebDAV file system cache files
 .DAV/
+
+# tags files (from ctags)
+tags
diff --git a/pint/default_en.txt b/pint/default_en.txt
index 4035883..fba87d2 100644
--- a/pint/default_en.txt
+++ b/pint/default_en.txt
@@ -72,6 +72,10 @@ cmil = 5.067075e-10 * m ** 2 = circular_mils
 darcy = 9.869233e-13 * m ** 2
 hectare = 100 * are = ha
 
+# Concentration
+[concentration] = [substance] / [volume]
+molar = mol / (1e-3 * m ** 3) = M
+
 # EM
 esu = 1 * erg**0.5 * centimeter**0.5 = statcoulombs = statC = franklin = Fr
 esu_per_second = 1 * esu / second = statampere


Test patch:
diff --git a/pint/testsuite/test_issues.py b/pint/testsuite/test_issues.py
index 3b06fd6..b5c494b 100644
--- a/pint/testsuite/test_issues.py
+++ b/pint/testsuite/test_issues.py
@@ -39,7 +39,6 @@ class TestIssues(QuantityTestCase):
 
     def test_issue29(self):
         ureg = UnitRegistry()
-        ureg.define('molar = mole / liter = M')
         t = 4 * ureg('mM')
         self.assertEqual(t.magnitude, 4)
         self.assertEqual(t._units, UnitsContainer(millimolar=1))
@@ -561,4 +560,4 @@ class TestIssuesNP(QuantityTestCase):
         a = np.asarray([1, 2, 3])
         q = [1, 2, 3] * ureg.dimensionless
         p = (q ** q).m
-        np.testing.assert_array_equal(p, a ** a)
\ No newline at end of file
+        np.testing.assert_array_equal(p, a ** a)