Merge branch 'feature/writers-improvement' into 'develop'

CMakeLists.txt

src/language/modules/WriterModule.cpp

src/mesh/Connectivity.hpp

src/mesh/Mesh.hpp

src/output/CMakeLists.txt

src/output/GnuplotWriter.cpp

src/output/GnuplotWriter1D.cpp

src/output/GnuplotWriter1D.hpp

src/output/VTKWriter.cpp

src/output/VTKWriter.hpp

src/utils/Messenger.hpp

src/utils/pugs_config.hpp.in

tests/test_Messenger.cpp