Adjust geometry to (xmin, ymin) or (xmean, ymean) before running mesh generation

[Huite]

Currently, the location of vertices are extracted and then directly provided to Triangle and Gmsh.

Depending on the coordinate system with small cell sizes, this means that very large values are computed to attain tiny differences. This doesn't obviously doesn't work well with finite precision.
We can get "free" precision by defining a new coordinate system, computing the mesh, and then restoring to the old coordinate system.

This should probably be done by default, with a property to control it; there may be cases where it results it gives trouble, if we lose precision due to addition and subtraction of the coordinates.