interpolation (which prevent geo transforms on extruded entities)

- display all control points in GUI for gmsh curves
- uniformize Fl_Value::step() values in GUI

instead of implementation details (tetgen+delaunay & co)

moment; we need to change the actual algorithm to ensure that we create
correctly oriented triangles in the first place)

added simple Helhmoltz solver + demo finite element plugin

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

moved all string options to std::string

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WARNING: this has the potential to break things in subtle ways...
         PLEASE TEST AND REPORT ANY WEIRDNESSES
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