Refs #9494. Store the current label instead of a pointer to ws group

The label gets updated when new data is loaded and is used for workspace
naming and grouping. This seems to be more robust than storing a shared
pointer and fixed the problem.
Refs #9494. Make the grouping function add to group if it exists

It makes it potentially faster and we can check for duplicates to avoid
endless warnings.
Refs #9494. Set the correct include path

Forgot to fix that when rebased the ticket onto the master
Merge branch 'bugfix/9494_muon_empty_group_name_after_fitting' into develop