Modify MERLIN_Definition.xml to use locations tag for tubes re #7967

This shortens the definition file considerably, but does increase loading time.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Correct pixel type for 2nd type of tube re #7967

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge remote-tracking branch 'origin/feature/7967_merlin_locations_tag'