Fix a bug in testmode re #6344

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Put LinearBackGround into slit/bar fitting re #6344

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Tidy up tube_calib.py code re #6344

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Reject peaks very short of expected height re #6344

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Take account of tube center re #6344

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>