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.
Re #8108. Fix and enable the last test.

As far as I can tell, setting the filename at this point in
ExperimentInfo is not necessary.