Create a fluorescence nexus file

[DominicOram]

As a user I would like to be able to analyse data that comes from the fluorescence detector. In #236 we found that we could write out data write out data from the detector as an h5 file. However, there is additional metadata, as described in the comments of this issue, that is needed for analysis to work. Currently analysis are reading this data from a config file in /dls_sw/ or directly from the information we send in #233. It would be preferable for us to produce a nexus file that is self contained with all the data that's required for analysis. We will at least need:

• Calibration data - likely stored as described in https://github.com/vasole/pymca/blob/caccc91650a4a6c79151dd02f3dd9dfb30b84279/doc/source/recipes/xrfhdf5stack.rst#L17
• Beamline energy

Acceptance Criteria

• A callback writes a nexus file for fluorescence data
• All data required for analysis is stored in the nexus file
• When triggering analysis we need only send the path to this file

[DominicOram]

FYI @pblowey