PCB / Protoboard Holder v2

This is a more sophisticated version of my original [mounting frame](https://www.thingiverse.com/thing:36576460).

This one is better because it allows you to have irregular spaced mounting pillars (e.g. only two or three) and it allows you to specify the number of fixing holes, and their offset with respect to each other.

The origin for the fixing hole is the center of the board, so if there is only a single fixing hole it will be centered. If there are more then one, they will be distributed evenly off centre by `fixing_hole_distance`.