Generic screw distance

The new Screw::get_distance(const Screw &rhs) method should give a possibility to have a checked calculation of the distance to another Screw. The check is necessary as the primitive dot product calculation has a degeneration with parallel lines.

This new method is also used for the acos3 function, which uses the dot product.

Additionally, in acos3, parallel lines to the reference could be identity with no distance at all resulting in an error by throwing an exception where non is expected. The exceptions were adapted and now used to detect identity yielding to no displacement (acos3(...) = 0+0s)