Fix duplicated enum between StateVariable and ResonantFilter

[tomcombriat]

This fixes #171.
@tfry-git and @sensorium if you comments on the naming please let me know.

Otherwise I'll merge it soon!

Cheers,
Tom

[tfry-git]

An alternative might be to share the same enum between both classes. I see no immediate downside to having one header including the other (ok, compilation will be a miniscule bit slower, but the compiled code should not be any different), so it would only have to be defined in one of them. This would avoid the cumbersome naming.

[tomcombriat]

Ah yeah, this came to mind at some point but was feeling a bit unconfident about for instance including ResonantFilter.h into StateVariableh. I guess compilation will be a bit longer, but the "unneeded" functions should not be included into the final executable right?

[tomcombriat]

@tfry-git It compiles fine.
(Sorry for three commits for one line, will try to squash them)
Have squashed them.