Add dodal device for i03 beamstop

[rtuck99]

Part of fix for DiamondLightSource/mx-bluesky#698

This adds a beamstop device to the i03 beamline.

See also mx-bluesky PR 698

Instructions to reviewer on how to test:

1. dodal connect i03 connects the beamstop
2. 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 97.48%. Comparing base (6e13d85) to head (e6ad77e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #970      +/-   ##
==========================================
+ Coverage   97.47%   97.48%   +0.01%     
==========================================
  Files         147      148       +1     
  Lines        6208     6235      +27     
==========================================
+ Hits         6051     6078      +27     
  Misses        157      157              

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[olliesilvester]

Looks good, more optional comments but feel free to ignore

[olliesilvester]

Maybe better to call these, eg, x_mm

[rtuck99]

I originally modelled this after the I24 beamstop which has identically named PVs but maybe we should change them here and also look at whether we should update that one as well.

[olliesilvester]

Yeah, personally I always prefer to include the units in names, so in I24's too

[DiamondJoseph]

[When emitting bluesky documents] the units will come through as part of the descriptor document Configuration, straight from the EPICS motor record if they're configured there- is there a reason you want the units in the names here?

[olliesilvester]

It's just much easier to write plans when you can immediately see the units in code

[rtuck99]

We've found quite a few cases where unit conversions have been the cause of a lot of confusion, areas of the code where we thought things were in one unit but actually in another. It's much easier to spot when a conversion up or down the unit scale is missing if signals and attributes are named clearly, and it's not a big overhead - if the unit changes, you have to change the code everywhere anyway so it's not a big deal.
It's a general preference across the team.

[olliesilvester]

We could reference DiamondLightSource/mx-bluesky#484 in this comment too