Coregistration and exporting it to BIDS

[Moo-Marc]

This is still a work in progress, a revised version of this previous closed PR: #511
I'm putting it here now to share it in its current state with users that also needed this functionality.
The main features are:

• The ability to replace the MRI anatomical fiducials with the digitized ones, meant to be done after automatic or manual coregistration. Currently added to the process: Import > Channel File > Adjust Coordinate System > "Replace MRI nasion and ear points with digitized landmarks (cannot undo)"
• A script (not yet accessible in the GUI) to export the coregistration in the original BIDS metadata files, based on the MRI anat fids. bst_save_coregistration.m

Note: This relatively old branch of mine contains other related changes, e.g. changes to generating the head surface and the automatic registration with head points, which would likely be removed from this PR prior to being ready for review.