Draft: Add commensurate lattice point function because I was bored

I added the commensurate lattice point function I wrote for the mode projection stuff.

It does what it does but in order to make a nice example I kind of need to know what type of data to start with and what you want. I.e. q-points or k-points, Cartesian or scaled etc.

prim_cell =  [[1, 0], [1, 1]]
super_cell = [[2, 0], [0, 2]]
lps = tools.get_commensurate_lattice_points(prim_cell, super_cell)
print(lps)
[[-1  1]
 [ 0  0]
 [ 0  1]
 [ 1  0]]

Related to #60 and #91 (closed)