Inconsistent unit cell parameter definitions in code and documentation

[chrisburke716]

The definitions of the angular unit cell parameters given at https://pscf.readthedocs.io/en/latest/unitcell.html are inconsistent with those used in the code.

For example, in a triclinic unit cell:

• The documentation lists alpha and beta as the angles between the b/c and c/a vectors.
• In the code (see unit_cell_mod/make_unit_cell, lines 456-458), alpha and beta are azimuthal and polar angles of the c vector. (though the comments in the code state a third, different definition for alpha.)