(This PR builds on #497, which is required for some of the functionality.)

This PR updates the __array_priority__ variable in the TransferFunction class that is used by numpy, as described in issue #459. With this change, multiplying a transfer function by an ndarray object gives a transfer function.

I also created a new unit test (control/tests/type_converstion_test.py) that goes through the matrix of interconnections in #497 and confirms that output type for each operation. In this current PR, no new functionality is included, but I did mark those entries that should (eventually) be updated. Depending on what people think about the PR, we can either leave those for another time or put them into this PR.