Week 6: Finished time-frequency example for multiple pipelines, wrapping up other PRs

This week I finished the tutorial and function dependencies for the time-frequency source space estimation https://github.com/mne-tools/mne-python/pull/10920. There were quite a few complications and disagreements about best implementation within the MNE core development group which made it a bit trickier and more time-consuming than originally thought, but it seems like it turned out better from having more opinions from which to make a consensus. I am a bit behind on the GUI code but I will do that this week. The main reason for that is refactoring parts of MNE to accommodate for the GUI which I think are hugely important for making it maintainable in the long run: I consolidated mixin classes so that MNE time-series objects have more consistent operations https://github.com/mne-tools/mne-python/pull/10945, this will be great for the TFRSourceEstimate class so that it can inherit these capabilities. The backend abstraction PR now is (almost) ready to merge after sorting through some tricky dependency issues and is a really nice way to make MNE better at a higher level than just making the GUI https://github.com/mne-tools/mne-python/pull/10913. Fortunately, I talked to Dan about these changes and how they fit into the broader class refactoring he is funded to do next year and so they will be well-aligned to do that. This should be great it making this project much more forward-facing toward the cutting edge of MNE and not a part lagging change that just creates extra work down the road.