fix warning
Fixes #296 and update #305:
 - implement constant per case in file @dolleg
 - implement FILE_INDEX support which needs support
update #305
 - implement FileIndex helper class to read/write FILE_INDEX
Uses #296 and #305 new features
up file_index work
Merge branch 'develop' into feature/ensight

Conflicts:
	testsuite/feeldiscr/test_mortar.cpp
up #326
 - start work on MPIIO in ensight gold format
 - for the moment use MACRO to select MPIIO mode
Merge remote-tracking branch 'origin' into feature/ensight
up #326
 - added mpi-io detection (I had this in a stash)
up #326
 - fix compilation
up #326
 it seems that SOURCES in TRY_COMPILE is only in recent version of cmake
up #326 add missing file
Fixing ensight gold exporter
Merge branch 'feature/ensight' of https://github.com/feelpp/feelpp into feature/ensight
Ensight Gold export:
- Fixed "No such file or directory" when file does not exists using
  in/out access (was preventing the writing of Variable files)
- Added code to avoid writting garbage to Ensight files
Feature/ensight:
- #353: Added options for merge domains (They will be merged in the main
  branch, once the feature/ensight is implemented)
- Updated with few comments
Fix for MPIIO detection
- Updated MPIIO implementation for Ensight Gold (geo files)
Issue #326 : Update MPIIO implementation
Issue #326:  Updated Ensight Gold with MPIIO:
- Start working on variable files
- Updated mesh file writing process