Attempt to load a default geometry from the RAW file if no xml definition available

[NickDraper]

LoadRaw should
- Extract the intrument name from the file
- Attempt to load the file from the instrument definition directory (should be defined in the config file)
- If not present then ti should use the L2 and 2theta values available in the RAW file to place detectors in a best guess position. This will allow the units change code to work without an instrument definition.

[NickDraper]

This issue was originally trac ticket 119