Make constraint regions easy to be defined by a user

[DomHazza]

Currently the sensors are placed anywhere in the rectangle defined by the borders. In the future when we use unfolded meshes with an irregular outline, or when the monoblock has a hole in it we need an effective way to specify only certain regions that the sensors can be placed at.

[DomHazza]

We need a way to define the bound of a region such that they are not a rectangle but an outline, and then a way to define constraints for the optimisation such that the sensors are only placed inside this outline, and we need to make sure the comparison positions are only chosen inside this outline.

[DomHazza]

We need to cope with irregular 2D meshes and with unfolding irregular 3D meshes into 2D meshes. I reckon the best way to do this is to update the functions in the Unfolder class in the sim_reader.