Add support for additional TFR methods

[tsbinns]

Currently, cwt_morlet is the only TFR method supported, and I see there is a note in epochs.py: # reminder to add alternative TFR methods.

Would it be possible to add support for e.g. the multitaper TFR method available in the mne-base package?

Best, Thomas!

[adam2392]

Yes that is a good idea. A PR is welcome! :) Lmk if you need guidance