The reorganisation of the option panel and the new mesh I/O layer are done.

Mesh I/Os currently available:

- Gmsh MSH format: read and write
- Gmsh POS format for mesh statistics: read and write
- INRIA MESH format: read and write
- STL: read (ascii and binary) and write
- VRML: read (partial) and write
- UNV: write

Everything is significantly faster than before (e.g. readMSH() can be more
than twice as fast when there are lots of physicals)