Add tests for checking dimensions

tests/froude_test.py

@@ -11,4 +11,7 @@ def test_froude_force():
     assert froude.model_to_proto(1, 10, 'N', 'N') == 1000
 
 def test_unit_dimensions():
+    assert froude.dimensions('m') == 'L^1'
+    assert froude.dimensions('kg') == 'M^1'
+    assert froude.dimensions('s') == 'T^1'
     assert froude.dimensions('N') == 'L^1 M^1 T^-2'