added simple Helhmoltz solver + demo finite element plugin

Removed GSL dependency. Gmsh now uses blas and lapack for linear algebra
stuff.

- fix bb computation to avoid model jumping around when adding points in z=0 plane
- pass interpol matrices as ref

multi-window & multi-coordinate system drawing.

There's no new functionality for now: PLEASE TEST THAT EVERYTHING
WORKS AS IT USED TO BEFORE.

reparametrization through harmonic maps enhanced