ENH: Add OTP processing option

[mdclarke]

Closes #355

[larsoner]

@mdclarke looks like it's close! I pushed some commits to fix the CIs. CircleCI was broken and Travis wasn't running, so I fixed those. Then in terms of your changes, I had to change the order of some stuff to make the YAML match the overview.rst.

I think what's left is:

1. Update the flowchart by changing https://github.com/LABSN/mnefun/blob/main/mnefun/_flow.py
2. Make it so that the SSS step uses either the raw files (if otp_dur is None) or the OTP'ed files (if otp_dur is not None)

[codecov-commenter]

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