Mbs analyser slits

[oliwenmandiamond]

Fixes #2069

Instructions to reviewer on how to test:

1. Check implementation makes sense
2. Check tests pass

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.14%. Comparing base (153fde4) to head (ec824b3).

Additional details and impacted files

@@              Coverage Diff              @@
##           mbs_detector    #2074   +/-   ##
=============================================
  Coverage         99.14%   99.14%           
=============================================
  Files               342      343    +1     
  Lines             13281    13339   +58     
=============================================
+ Hits              13167    13225   +58     
  Misses              114      114           

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