Fixed errors passing Numpy int64 values for int arguments (#2961)

wrappers/python/src/swig_doxygen/swig_lib/python/features.i

+%begin %{
+#define SWIG_PYTHON_CAST_MODE
+%}
+
 %include pythoncode.i
 %include exceptions.i
 %include extend.i

wrappers/python/tests/TestNumpyCompatibility.py

@@ -91,6 +91,12 @@ class TestNumpyCompatibility(unittest.TestCase):
         self.assertEqual(size, 10)
         np.testing.assert_array_almost_equal(energy, np.asarray(energy_out))
 
+    def test_int64(self):
+        indices = np.array([0,1,2])
+        sys = mm.System()
+        sys.addParticle(2.0)
+        assert sys.getParticleMass(indices[0]) == 2.0*unit.amu
+        
 
 @unittest.skipIf(NUMPY_IMPORT_FAILED, 'Numpy is not installed')
 class TestNumpyUnits(unittest.TestCase):