nabu-config improvements

Things to improve:

• For EDF datasets, when there is no trailing / in the dataset name, the reconstructed filename goes at the folder root by default. Otherwise, the file name is _rec_0000.hdf5, which is not good either.
• nabu-config --validate (instantiate a ProcessConfig and so on, without actual processing)
• Parameters for nabu-config --bootstrap: --dataset, --phase, etc
• nabu nabu.conf should automatically deal with the delta_z parameter (based on available memory)