Add GetFEM to the Finite Elements list

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GetFEM is similar to FEniCS, Firedrake and FreeFEM, already in the list, but it has its own strengths:

• Good support for modelling of contact problems
• C++ API, as well as support for several scripting languages Python/Octave/Matlab
• A weak form language which is isolated from the programming language
• Support for XFEM and for working with non-matching meshes in general

GetFEM is easy to try out, but also suitable for creating large models:

• is available on Debian/Ubuntu/Docker
• has large collection of examples
• imports meshes created with gmsh, gid or ANSYS
• supports parallel distributed assemblies with OpenMPI
• it is one of the oldest public FE codes (over 20 years as open source), but it has been continuously maintained and modernized