Mathematical simplification in hpix_mollweide_xy_to_angles.

sqrt((1-v) * (1+v)) -> cos(asin(v)) (since asin(v) already needs to be calculated).
New (empty) functions hpix_calc_angular_distance_from_vectors and hpix_calc_angular_distance_from_angles

Test cases are already in place, but both functions are not implemented yet.
Missing file added
Unused variables/operations (caught by the Clang static analyzer) removed