Add HDF5 Subfiling feature to DECL'HDF5 plugin

[MelineTrochon]

To improve Gysela checkpointing, an idea is to use the subfiling feature from HDF5.
But it's not accessible from the DECL'HDF5 plugin.

[Yushan-Wang]

To improve Gysela checkpointing, an idea is to use the subfiling feature from HDF5. But it's not accessible from the DECL'HDF5 plugin.

What would be the specific need in terms of subfiling configuration? Do you need to have the possibility to set one-file-per-proc, one-file-per-node in the yaml? Or a simple total-subfile-number will be sufficient?