XRC: Ensure easy way to change box sizes and exposure time

[DominicOram]

It may be useful to oversample the XRC and take data with smaller box sizes (even though the aperture is bigger than this). This should give us a more precise location for the centre of mass.

Acceptance Criteria

• There is an easy way for the user to change box size and exposure time
• We communicate how to do this to the user
• This does snot affect aperture selection
• We confirm on the beamline that we still go to the correct place even with smaller box sizes

[dperl-dls]

• easy means: change it in only one place, and documented, and where relevant people have permission
• we have never changed it before, needs testing
• either we can:
  • supply this with the xray centring,
  • or point neil to CONST.PARAM.GRIDSCAN.BOX_WIDTH_UM,
  • or put that in config service and point neil there

will go with option 2, and make new issues for option 3 (including validating numerical input)

we think small to medium

[DominicOram]

Wants testing with beam

[DominicOram]

Tested in #211. Failed to work easily. Box size isn't just changed by changing BOX_WIDTH_UM because it isn't passed through to grid_detection_plan. Trying to hard code in that call resulted in https://ispyb.diamond.ac.uk/dc/visit/cm37235-3/dcg/12681792 , which failed to kickoff the FGS. Some offline time needed to investigate