refs #9438 Current state of LET instrument folder.

Redundancies are present and some regularization is necessary.
refs #9438 Better diagnostics

During IDF changes, it was found difficult to understand what is going wrong when monitor number is specified incorrectly. This change should help to address similar problem in a future.
refs #9438 Should fix duplicated IDF issue.
refs #9438 Found old dr2to& parameters file smuggled from LET
refs #9438 returning missing changes

which were lost from copying IDF and parameter files from mantid 3.0
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.