Re #9545. Set internal monitor workspace pointer.

This is in addition to the existing ways of adding the workspace
separately to the ADS with a "_monitors" suffix and returning
it as an output property. This will allow the new way to be used going
forward without breaking the old way.
Re #9545. Add setting of the monitor workspace pointer.

This is in addition to the existing way of an additional ADS entry with
"_monitors" suffix. It allows clients to use ExtractMonitorWorkspace on
the main output workspace and not need to worry about the name of the
workspace (it will work if LoadPreNexus was run as a subalgorithm, which
is not the case for old way).
Re #9545. Doxygen fix.
Merge remote-tracking branch 'origin/feature/9545_loadeventnexus_add_internal_monitor'