ENH gtgram: add f_max parameter

[christianbrodbeck]

Hi there, good to see this is on PYPI now. I am transferring this PR from the original repo (detly#11) where @Borda already approved it

This adds an f_max parameter to gtgram(). This parameter was already in the documentation but missing from the implementation.

[Borda]

Great addition, shall we a test?

[christianbrodbeck]

Added a simple test for centre_freqs(); would be more involved for gtgram() because test are parametrized with a dataset test_gammatonegram.py. However I could add a new test picking one entry in the dataset and comparing results when called with or without f_max?