don't load 'fallback' parameter file if map in nexus. re #9467

Change the behaviour of LoadIDFFromNexus and before doing this,
as part of trying to ensure I wasn't broking something, updated
various code comments related to loading of xml files
Updated alg doc so it reflect what is done in code. re #9467
Merge remote-tracking branch 'origin/feature/9467_if_param_map_in_nexus'