unified class Incompressinle for 2d and 3D problems

Several things for multi-constraint mesh partitioning + start circumscribed circle of MElement (need to be finished)

python interface: all std::vector<> in element constructors are declared as const, allows easy python binding

Fracture can be prescribed by a different law on interface
Explicit system is implemented with petsc vector operations

AnalyseCurvedMesh development (works for triangle and tetra but is not yet optimized and is still incomplete, edit PluginManager.cpp to activate the plugin)

* basic Abaqus INP mesh export

* issue error for zero volume elements in FEA export
* reset element number between 2D/3D in FEA export

Compute radius of the inscribed circle within an element. Done for Mquadrangle, MTriangle, Mtetrahedron

pretty please do not just "throw;" when you don't know what to do

+ constify MElement some more

- renamed functionSpace into polynomialBasis (functionSpace will be much higher level)

added some more topological infos in high order elements (useful for DGM,
still have to be tested)

renamed most gmsh* classes