Improve design of writes

- WriterModule now just contains calls to writers
- Add a WriterBase class to share cleanly common functionalities

Also improve VTKWriter functionalities
- the time series file (`.pvd`) is now produced and should be loaded
to get proper data

Finally add a GNUPlotWriter
- works for both 1D and 2D
- cannot mix node and cell data in the same result file (this is done
on purpose)
- A simple preamble describes the content of the file