Write an OPM tutorial for CuttingEEG X

[robertoostenveld]

I discussed with @contsili that for the CuttingEEG X at the Donders we want to make a mash up of the existing OPM tutorials.

The plan is something like this, although Konstantinos might remember it slightly differently:

• do a quick ERF analysis of SQUID MEG, show normal multiplot+topoplot
• make ft_plot_topo3d figure at specific latency (for later comparison) together with head surface from coregistered MRI
• process multiple runs of OPM data
• timelockanalysis, appendtimelock, multiplot+topoplot
• explain artifact rejection per run, repeat over runs and again work towards appended topoplot
• append the raw data, inserting nans for the missing channels in each run
• show block-wise matrix rejectvisual, explain that spatial and temporal information does not allow for overall ICA or HFC
• explain that simultaneous measurement of all (or many) channels gives more options for denoising (artifact removal)
• explain artifact removal per run, repeat over runs and again work towards appended
• coregistration with anatomical MRI
• make ft_plot_topo3d figure from OPM ERF at specific latency together with head surface from coregistered MRI

[robertoostenveld]

I added "cuttingeegx" to the workshop overview page, made a landing page, and added a placeholder for the actual tutorial.

[contsili]

I opened a PR and I started working on the tutorial. See #813