MV: add get_neuromag_trans to the coreg module

get_neuromag_trans is used to transform points from their space to the
neuromag RAS space.
RENAME

rename get_neuromag_trans to get_ras_to_neuromag_trans.

fix breaking the test_coreg.py
FIX

Global replacement
Merge pull request #852 from t3on/tb-coreg

MV: add get_neuromag_trans to the coreg module