Write a coregistration tutorial for OPM data

[robertoostenveld]

as discussed with @tc-fieldline and @schoffelen.

This is to include

• https://github.com/fieldtrip/fieldtrip/projects/9#card-91155263
• https://github.com/fieldtrip/fieldtrip/projects/9#card-91155255
• https://github.com/fieldtrip/fieldtrip/projects/9#card-91155267
• https://github.com/fieldtrip/fieldtrip/projects/9#card-91155259

A read-only version of the draft tutorial can be found here https://hackmd.io/@robertoostenveld/H1BzNlcUT

[robertoostenveld]

this has been completed with http://fieldtriptoolbox.org/tutorial/coregistration_opm/