refs #8480 just created branch
refs #8480 changed nexus file holder to scope pointer

as it was locking file on failure (and probably caused memory leak)
refs #8480 Should fix the issue, but testing is needed
refs #8480 fixed unit tests compilations
refs #8480 Fixed MDEvents test
refs #8480 final changes should fix the issue
Merge branch 'bugfix/8480_LoadMDafterMergeMDFiles' into develop