Fix unit conversion for grid scan step size

[DominicOram]

Fixes DiamondLightSource/mx-bluesky#220

Instructions to reviewer on how to test:

1. Confirm tests still pass and units are sensible

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 95.97%. Comparing base (7f7b7a6) to head (5475baf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
+ Coverage   95.85%   95.97%   +0.11%     
==========================================
  Files         136      136              
  Lines        5576     5566      -10     
==========================================
- Hits         5345     5342       -3     
+ Misses        231      224       -7     

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[dperl-dls]

lgtm

[rtuck99]

Approved