add BESA .elp support to read_montage

Direct translation of EEGLAB routines for reading spherical BESA .elp channel location files.
Not to be confused with polhemus .elp files (no overlap with https://github.com/mne-tools/mne-python/pull/1001).
Possibly need smarter way of handling header files; the only example I know has 1 header line, but there may be other formats.