ENH: Allow multiple tasks to be processed jointly

[larsoner]

Before merging …

• Update type defs
• Updated docs/source/dev.md.inc
• Handle epochs-based cov somehow... probably by making sure baselines are the same and compatible and if not, tell the user to use a custom function instead of "epochs"
• Ensure report section titles only get task added if there is more than one task
• Add processing of multiple tasks jointly (tested locally with dataset that needs it)
  • Initial
  • Preproc
  • Evoked
  • Source est
• Ensure all report sections are tagged by task type
• Ensure preprocessing/* uses union of all bad channels across all tasks (including noise!): Dropped in favor of existing ENH: Add mf_bads option #760
• Get CIs green
• Add dataset that demonstrates this capability

Just opening to make sure nothing is broken yet. It shouldn't be since this should just relax type def checks in a way that shouldn't matter, but 🤷

Closes #820

[larsoner]

Okay, I think OpenNeuro is having a bad day. But the datasets that can be downloaded work, and the multi-task support works on my dataset!

@drammock do you want to review+merge now, or wait until we can get a suitable dataset for testing?

[drammock]

I won't have time to review today, but I do have permission to use one subject from the PREK dataset (and I have the files), so maybe worth waiting for that. I hope to get to it early next week.

[larsoner]

Okay feel free to push commits directly to this PR. It'll be a good way to test it out! Or I can do it once the data are uploaded