Sort out output file generation

[leobpujol]

Currently, output can be generated in gnuplot, tecplot, vtk and hdf5 formats.

VTK was updated with newly implemented parameters (infiltration, porosity, manning_beta, ...)
-> TODO : troubleshoot VTK outputs depending on selected options (use_infiltration, ...)

-> TODO : update tecplot and gnuplot format if needed

-> TODO : clarify use of HDF5 format, setup working example in tutorials