Fix another small bug for unit handling with numpy arrays. Fixes the exception:

Traceback (most recent call last):
  File ".../python/tests/TestNumpyCompatibility.py", line 87, in testNumpyAttributes
    d = self.data.reshape((100, 3))
  File ".../simtk/unit/quantity.py", line 575, in reshape
    return Quantity(self._value.reshape(shape, order=order))
  File ".../simtk/unit/quantity.py", line 142, in __init__
    if value == first_item:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Also add a test for new unit numpy capabilities.

wrappers/python/simtk/unit/quantity.py

@@ -139,7 +139,15 @@ class Quantity(object):
                         first_item = iter(value).next()
                         # Avoid infinite recursion for string, because a one-character
                         # string is its own first element
-                        if value == first_item:
+                        try:
+                            isstr = bool(value == first_item)
+                        except ValueError:
+                            # For numpy, value == first_item returns a numpy
+                            # array of booleans, which cannot be evaluated for
+                            # truthiness (a ValueError is raised). So in this
+                            # case, we don't have a string
+                            isstr = False
+                        if isstr:
                             unit = dimensionless
                         else:
                             unit = Quantity(first_item).unit

wrappers/python/tests/TestNumpyCompatibility.py

@@ -78,5 +78,24 @@ class TestNumpyCompatibility(unittest.TestCase):
         assert size == 10
         np.testing.assert_array_almost_equal(energy, np.asarray(energy_out))
 
+class TestNumpyUnits(unittest.TestCase):
+
+    def setUp(self):
+        self.data = unit.Quantity(np.arange(300), unit.nanometers)
+
+    def testNumpyAttributes(self):
+        d = self.data.reshape((100, 3))
+        self.assertTrue(unit.is_quantity(d))
+        self.assertTrue(unit.is_quantity(d.sum()))
+        self.assertTrue(unit.is_quantity(d.sum(axis=0)))
+        self.assertTrue(unit.is_quantity(d.std()))
+        self.assertTrue(unit.is_quantity(d.std(axis=0)))
+        self.assertTrue(unit.is_quantity(d.max()))
+        self.assertTrue(unit.is_quantity(d.max(axis=1)))
+        self.assertTrue(unit.is_quantity(d.min()))
+        self.assertTrue(unit.is_quantity(d.min(axis=0)))
+        self.assertTrue(unit.is_quantity(d.mean()))
+        self.assertTrue(unit.is_quantity(d.mean(axis=1)))
+
 if __name__ == '__main__':
     unittest.main()