refs #9438 fixing system test: Renamed LET_Definition_dr2to7

to LET_Definition

and LET_Parameters accordingly, as LET2014Multirep nexus system tests source files contains old LET_Definition and LET_Parameters, which do not work with recent reduction. The only way to run these files is to reload LET_Definition.xml and LET_Parameters.xml
from the IDF.
Merge branch 'feature/9438_LET_IDFchanges' into develop