Add support for inference of method arguments

This MR closes #409 (closed).

• A new test has been added to ensure that method arguments are inferred correctly, complementing the same functionality for functions
• A new function is provided, utils._infer_args_from_function_except_n_args to allow an arbitrary number of arguments to be ignored.
• utils.infer_parameters_from_function has been updated to infer if a passed argument is a function or a method (or will raise a ValueError if it is neither.

The behaviour of utils.infer_parameters_from_function may not be considered optimal, as it will fail in the case that an object with a __call__() method is passed as an argument. If you're keen for this form to be supported I can add an additional case for it.

test/utils_test.py

@@ -80,6 +80,12 @@ class TestInferParameters(unittest.TestCase):
         del self.source1
         del self.source2
 
+    def test_args_inference_method(self):
+        """Test that inference of arguments from methods ignores ``self``"""
+        expected = ['a', 'b']
+        actual = utils.infer_parameters_from_function(self.source2)
+        self.assertListEqual(expected, actual)
+        
     def test_args_kwargs_handling(self):
         expected = ['a', 'b']
         actual = utils.infer_parameters_from_function(self.source1)