Issue #474:
- Cmake:
* Added LibXml2 libs for compilation (the package was already looked
 for, but the libs were not added)
* Check for the parallel components to be present with VTK and set a
 flag accordingly
- Source:
* Restricted the use of MPI code to VTK >= 6: reduces code complexity
* Added pvd file to directly load all the timesteps
* Cleaned up code and
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop