refs #4614 here the changes brought back again
refs #8614 trying to nail down reason for MARI tests failing
refs #8614 Now it works as expected for LET

and changed LET_Parameters.xml to have default nexus files to be nexus, not event nexus files.
refs #8614 Minor comments
refs #8614 Removed unnecessary parameter in instrument definition file

Generic loader now distinguish between nexus and event nexus file.
Merge branch 'bugfix/8614_letReduction' into develop