LoadDetectorInfo now just loads parameters & adjust det positions

Refs #8640
Move MARI det parameters to instrument level

This helps the performance of the LoadDetectorInfo algorithm.
Also fixed a spelling error in LET_Definition.xml comment
Refs #8640
Swap out UpdateInstrumentFromFile for LoadDetectorInfo

This will allow users to overwrite the delays/pressures/thicknesses
if required.
Refs #8640
Paralleize creating vectors from the NeXus file.

Also kill a warning from the test.
Refs #8640
Merge branch 'feature/8640_restore_loaddetectorinfo_removal' into develop

Conflicts:
	Code/Mantid/scripts/Inelastic/CommonFunctions.py
Refs #8640