[WIP] Add mne-connectivity dependency and wrap spectral_connectivity_time there

[adam2392]

This is still a WIP, but adds mne-connectivity to certain places in the docs, CI and does an initial simplification of conn_spec to just call underlying mne-connectivity function.

Related to: #24 and https://github.com/mne-tools/mne-connectivity/pull/71

This PR should reveal how much disparity there is between the current implementation of connectivity containers in mne-connectivity vs frites.