Add a generic hook class to output data using the `FieldsIO` helpers + demo + documentation.

[tlunet]

Following discussion in #533. Here is a first TODO list :

• [ ] Description of the base native data IO structure in the documentation
• [ ] Hook class to use the FieldsIO
  • [x] base serial implementation
  • [x] MPI setup
  • [ ] demo and documentation
• [ ] Add an option for multiprocessing in Rectilinear.toVTR, which default to one proc
• [x] Put back the *_ALL in MPI-IO interfaces for Rectilinear

[tlunet]

But then, one critical question : if we make the FieldsIO classes the base IO feature of pySDC, shouldn't we make them compatible with python 3.9 and 3.10 ?

[tlunet]

Adaptation to python 3.9 and 3.10 + full collective IO done with #538