Reparam rewrite

Rewrite of the reparametrization code for better integration and use of faster linear solvers.

@Celestin @pbeaufort @arthurbawin @Remacle: from my point of view once this has been thoroughly tested we can remove contrib/hxt/reparam (but see below).

@arthurbawin: your automatic mesh size field still currently depends on hxt_curvature. We discussed with @Remacle and I would like to help with a tighter integration in Gmsh, as we will use p4est for other things in the future and duplicating p4est interfaces is not optimal. Can you get in touch with us 2 so we can discuss this? Thanks.