ENH: Make kit2fiff work with Polhemus digitizer files

[TalLinzen]

mne.gui.kit2fiff can only deal Fastscan points + headshape files at the moment. The C version of kit2fiff was able to deal with older Polhemus .elp and .hsp files (though not with Fastscan files). We have a lot of older data sets that were digitized using the older Polhemus device so it would be nice to be able to read them in as well.

I've discussed this with @t3on and he thinks that it should be doable. For NYU at least this is mostly for legacy support (data sets recorded in 2012 or earlier), so it might not be worth emulating the C version if it's a huge amount of work.

(Thanks Teon!)

[teonbrooks]

I got the reading function to work now I need to make sure I understand the transformation properly.

@christianmbrodbeck So the legacy elp and hsp are already transformed in the space where they have been scaled to meters and all the points lie in the same plane (z=0), where the X-axis is the through the nasion, and the Y-axis goes through the left (negative-Y) and right (positive-Y) peri-auricular points, so it's Anterior (X), Right (Y), Inferior (Z) for the positive axes. This would mean that to go to Neuromag space from here, we would only need to do the following [X,Y,Z] => [-Y, X, Z] or do we need to remove this transform first then go the transform route that we have established?

[teonbrooks]

PR #1001 opened. Closing this issue.

[christianbrodbeck]

[X,Y,Z] => [-Y, X, Z]

If it's "Anterior (X), Right (Y), Inferior (Z)" you'd want [Y, X, -Z] to go to RAS space