Re #8108. Generalise where to look for instrument.

Remove the hard-coding pointing to the ISIS place in favour of the
variable that's used everywhere else.
Re #8108. Use the LoadEventNexus loadInstrument static method.

As well as reducing code duplication, this method tries to load the
instrument from the nexus file if possible.
Re #8108. Refactor to correctly load from SNS ADARA files.

While also continuing to work for the other files that have the
instrument XML inside the nexus file (processed, ISIS files).
Re #8108. Add .nxs.h5 to list of file extensions.

Also fix some formatting.
Re #8108. Add loading of the parameter file to LoadIDFFromNexus.

However, the algorithm doesn't fail if the parameter file is missing.
Merge branch 'feature/8108_load_inst_from_adara_nexus_files' into develop