Adds unit test for improver.api

improver_tests/test_api.py

@@ -0,0 +1,8 @@
+from improver import api
+import pytest
+
+
+@pytest.mark.parametrize("module", (api.PROCESSING_MODULES.keys()))
+def test_all_values_are_callable(module):
+    """Checks that each item in the PROCESSING_MODULES dict points to a callable object"""
+    assert callable(getattr(api, module))