Fixes after waffle testing

[DominicOram]

Fixes DiamondLightSource/crystallography-bluesky#14

The Union issue on the linked issue was due to the Attenuator not being able to have a union of types in it's set method. I spent some time trying to find ways around this to allow the device to be set with floats/ints but ended up leaving it so you had to use the enum. This is the most type correct way for it to work but we'll see how users react to it...

Instructions to reviewer on how to test:

1. Confirm new tests pass
2. Confirm all other problems in the linked issue are covered by other issues

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 99.11%. Comparing base (e4914d5) to head (d62fa3d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1978   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         318      318           
  Lines       12221    12224    +3     
=======================================
+ Hits        12113    12116    +3     
  Misses        108      108           

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