refs #9061 This brings new LET configuration files to Mantid

and fixes the issue but the ticket can be merged to master/developer only after #8614
refs #9061 Should fix system test

and the issue with monitors taken from incorrect instrument parameters file
refs #9061 should fix issue with LET configuration

though problems with loadEventNexus and CopyInstrumentParameters are apparent.
refs #9061 Minor comments & formatting
Merge remote-tracking branch 'origin/feature/9061_newLetConfig'