mx-bluesky#1765 Refactor grid parameters

[rtuck99]

Part of fix for:

• box_size_um is required by GenericGrid even though x + y box dimensions can be specified independently mx-bluesky#1765

See also:

• 1765 Refactor grid scan params mx-bluesky#1787

This removes grid_position_to_motor_position(). The function is now part of xrc_result_utils in mx-bluesky.

Instructions to reviewer on how to test:

1. 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 99.09%. Comparing base (700c0b3) to head (6e61e48).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2112      +/-   ##
==========================================
- Coverage   99.16%   99.09%   -0.07%     
==========================================
  Files         347      347              
  Lines       13588    13588              
==========================================
- Hits        13474    13465       -9     
- Misses        114      123       +9     

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