Add code to check for unrecognized keywords associated with default settings

As pointed out in #526, it is possible to pass unrecognized keywords to `StateSpace` and not get an error message.  For example:
```
ct.StateSpace(A, B, C, D, remove_useless=True)
```
(the actual keyword should be `remove_useless_states`).

Similarly,
```
ct.set_defaults('statesp', remove_useless=True)
```
should generate an error.