DigMontages: information on coordinate systems; and adding a simple rescaling method

[sappelhoff]

I just got some sensor positions in a text file that are in the "CTF" coordinate system. Naturally, I wanted to turn this into a DigMontage. However, when trying to find an appropriate value for the coord_frame parameter, I was faced with two choices:

1. ctf_meg
2. ctf_head

I could not easily find any documentation on the differences between these two coordinate systems. Ideally we would have a page where each coordinate system that we support is described with mentioning at least the origin, and the direction of the three axes (x,y,z) with reference to fiducials/anatomical landmarks, wherever possible. What do you think?

Furthermore, I think mne.channels.DigMontage should get a method called .transform_to (or similar) that takes as input the string label of any supported coordinate system in mne. Alternatively (or additionally), this functionality could also become a parameter upon instantiation of the DigMontage. .apply_trans seems a bit tricky to use for that. Opinions on this?