1. IDS mangling used for checksum is now inst name + sha1 of xml
 1.1. sha1 is computed after converitng to linux line endings and trim
 1. VTP file names changes to be same filename as IDS mangling
 1. VTP save directory moved to appdata/mantidproject/instrument/geometryCache
 1. VTP file saving simplified as files based on a checksum cannot be out of date
 1. unit tests updated to account for this
 1. unused methods removed from IDFObject
 1. InstrumentDefinitionParser changed to initialise during constructor
 1. XML parsing changed to lazy load rather than during initialisation.

Still to do:
 1. cache SHA1 checksum if needed
 1. Change the filename reported for nexus files
 1. Manually test
 1. Stop saving the filename to nexus files
 1. update documentation

re #11818