allow calibration and ctc files to be None

[harrisonritz]

Trying to push Cerca OPM data through the pipeline.

We don't have calibration or ctc files for our OPM system. IIUC Seems like both are necessary, because we 1) estimate sensor orientations during coreg (so raw.info should be 'correct') and 2) we only use magnetometers.

The MNE maxwell functions allow for calibration and cross_talk to be passed as None, so should be allowed here as well.

Before merging …

• Changelog has been updated (docs/source/dev.md.inc)

[welcome]

Hello! 👋 Thanks for opening your first pull request here! ❤️ We will try to get back to you soon. 🚴🏽‍♂️

[larsoner]

Seems reasonable to me! Failures look real/related now, can you look? For example

https://app.circleci.com/pipelines/github/mne-tools/mne-bids-pipeline/4945/workflows/97316c87-6261-4295-ba6c-7d12c37cf15b/jobs/76881?invite=true#step-104-2857_39

[harrisonritz]

I had to remove mf_ctc_fname & mf_cal_fname from in_files, and load them through the maxwell_filter.
This was already the approach in find_bad_channels_maxwell.

[harrisonritz]

looks like this is passing 👍

[harrisonritz]

ok, this implements mf_cal_missing + mf_ctc_missing, and adds a new test for them.

[drammock]

I pushed a commit for a change that was proving tricky to pull off in the GH diff editor. Other comments are below.
Note that I've intentionally tweaked the config docstrings differently: it's a bigger deal for MEGIN systems if the crosstalk file is missing, than if finecal is missing. Happy to change that though if @larsoner disagrees.

I'll approve the CIs now and see how we do!

[harrisonritz]

looks good, thanks!

[larsoner]

Committed @drammock 's suggestions and marking for merge-when-green, thanks in advance @harrisonritz !

[welcome]

🎉 Congrats on merging your first pull request! 🥳 Looking forward to seeing more from you in the future! 💪