Issue #474:
- Fixed bad export of pvd meta-files. Data grouping through this files
  now works; File is correctly removed at each simlation restart;
- Fixed bad meta-file generation when using XML Multi block parallel writer (writing exclusion must be done manually);
- Fixed meta-file non parallel version.

Issue #475:
- Fixed transmission of time steps to Paraview through Catalyst.

Other:
- Added a define in timeset.hpp to remember the index of the first time
  step.
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop