Add an interface for LSQing

[pemsley]

As discussed 15th May.

[pemsley]

# start a new set of matches
mc.clear_lsq_matches()
# add some matches (typically only one match)
mc.add_lsq_superpose_match('A', 1, 3, 'B', 1, 3, 1)
mc.add_lsq_superpose_match('A', 11, 20, 'B', 11, 20, 1)
mc.add_lsq_superpose_match('A', 101, 103, 'B', 105, 107, 1)
# generate the matrix and apply it to the atoms of imol2
mc.lsq_superpose(imol1, imol2)
# now update the bonds for imol2...