Defer checking of Xmin value re #2027

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Fix CropWorkspace::getXMin problem re #2027

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Update comment re #2027

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Merge branch 'feature/2027_crop_workspace' into develop