write an OPM tutorial on software-based cleaning, amm/hfc/dssp/sss

[schoffelen]

We may be able to use some data from Fieldline for this, once confirmed I will get cracking (and also use the data for a test function)

[contsili]

I am using FieldLine OPM data for the CuttingEEGX tutorial (see #798). The plan is to share my FieldLine data in Fieldtrip's download server. Maybe you could use my data for your tutorial?

[robertoostenveld]

The cleaning needs to be done prior to appending, and AMM and SSS won't work very well on the small 32-sensor subset recordings that you have in every run.