Improve ability to work in detector coordinates

[Martyn Gigg]

Mantid does not currently offer many mechanisms to work with detector coordinates.

Both Pascal Manuel & Leighton Coates expressed interest in this area although it is not yet clear what their requirements are.

Gather requirements & allow a 2 week timebox for implementation

[Nick Draper]

Batch modify all SSC tickets to critical priority (this will also show up as an update for all those already as critical)

[Nick Draper]

Check with Pascal if this is the same as #11227

[Stuart Campbell]

This ticket has been transferred to github issue ​12065