Export integrated dataset in (yet another) NexusFormat ...

[kif]

Implement output to Nexus format, probably in pyFAI.io:

• [ ] https://manual.nexusformat.org/classes/applications/NXazint1d.html
• [ ] https://manual.nexusformat.org/classes/applications/NXazint2d.html
• [ ] All metadata should be available within the pyFAI.containers.Integrate1|2dResult instance
• [ ] Signature should take an index position within the stack and optionally a stack-size.
• [ ] Provide export test to validate both the creation of the HDF5 data-structure and the append of new frames.
• [ ] Ensure there is an option to save in addition the 4 accumulators (signal, variance, norm and norm²) for re-binning purposes.

Discussed with Mads Jørgensen & Frederik Holm Gjørup