Write an OPM tutorial for CuttingEEG X

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