Support for corotational finite elements. The class hierarchy of FEM elements requred some changes.
Now the tetahedron class supports large rotations (this is not yet implemented for hexahedrons, though).
Also, Rayleigh damping added in ChContinuumElastic material, and damping is supported in elements.
Improved the FEM experimental code. Some bugs corrected. Per-element-mass not yet working because a rework on the API of ChPhysicsItem is needed.
Major change in the interface to LCP solver technology.
Previously, solvers had a boolean flag in Solve(..) to ask that the [M]*v_old  term was added to the known term (forces, etc.) .
Now this is not needed anymore, because the LCPprepare_load() function in ChSystem will take care of this, thank to the new
functions VariablesFbIncrementMq() that have been added to all ChPhysicsItem objects.
The reason: this is required for the new FEM technology in unit_FEM.
Fixed bug in FEM experimental code ChElementBar and ChElementSpring (they worked only in horizontal direction).
Updated the test code for the FEM elements.
Fixed minor stuff in ChTriangleMeshShape
Added the method Set_A_Xdir for setting a rotation matrix by passing a direction, with Gram-Schmidt orthogonalization (previously this required calling the same function, but in ChVector, and it was more typing..)ù
Improved ChTriangleMesh geometry class.
New class for converting FEM stuff into visualization assets. Still experimental.
Improved FEM code.
Now all elements are corotational, including the hexahedrons.
Also, few bugs corrected.
Fixed problem with GNU compiler: it gave errors like "error: extra qualification ‘chrono::ChPythonEngine::’ on member ‘GetFloat’ [-fpermissive]" on ChPython.h, but the MSVC compiler was more tolerant so I haven't noticed it in the past.