ENH: centralize polhemus data readers

Fixed their references
ADD test for polhemus readers
Remove specific hsp/elp reader

This parsing is easily done with np.loadtxt. Code updated accordingly.
FIX _set_dig_neuromag for _kit2fiff_gui
Check shape when reading file
PEP8
ADD set_dig_points
FIX import, minor errors
UPDATE test_set_apply_dig_points
ENH: address shape check
FIX set, COSMITS
ADD SetChannelMixin
ADD test to mixin, FIX functions
FIX set
ENH increase coverage
ENH: Auto channel selection, FIX bugs
ENH simplify assignment, PEP8
ENH: remove unnecessary computation
ENH restore hsp warning
RM read_dig_points