GSOC Cleanup: Add EpochsTFR support to spectral_connectivity_epochs() and spectral_connectivity_time()

[tsbinns]

Discussed in call with @wmvanvliet & @larsoner. Most likely I would not be able to address this until ~November.

Part of the GSoC project was to add tools for generating surrogate connectivity data (#223). As this acts by shuffling Fourier coefficients, part of the work involved adding EpochsSpectrum objects as a valid data type for spectral_connectivity_epochs() (#220). However, this only addresses the Fourier/Welch and multitaper spectral modes of spectral_connectivity_epochs(), ignoring the Morlet wavelet approach, as well as spectral_connectivity_time() entirely.

To finalise the GSoC work, support should be added for EpochsTFR objects in spectral_connectivity_epochs() and spectral_connectivity_time(). Thankfully this should be easier than adding EpochsSpectrum was (mne-python#12747), since EpochsTFR already support Fourier coefficients.

This would also go some way towards addressing #126 (only 1.5 years later...), since you could pass an EpochsTFR object with time-freq. Fourier coeffs. from the multitaper mode into spectral_connectivity_epochs().