ApertureScatterguard now set using a well defined enum

[DominicOram]

Fixes #740

Instructions to reviewer on how to test:

1. Confirm interface to ApertureScatterguard is cleaner
2. Confirm tests still 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 ✅

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Additional details and impacted files

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  Files           ?      114           
  Lines           ?     4588           
  Branches        ?        0           
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  Hits            ?     4333           
  Misses          ?      255           
  Partials        ?        0           

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[dperl-dls]

See comment on the other PR, otherwise looks good