class AttenuatorMotorPositions: cleanse obsolete naming

[CoePaul]

• Internal naming conventions within the Attenuator motor positions class had been left in prior state before recent rename ( where the rename relfected a widened scope from purely demanding motor moves to also capture live positions of motors )

  The resulting obsolescene meant names of methods and internal variables
  did not reflect the wider usage scope - that is hereby fixed

• Also make internal merge of discrete indexed positions and continuous motor axis positions into a cached property saves the merge being recalculated - and makes its evaluation lazy

• Update tests to reflect renaming

• Improve tests with parametrized test cases

• Fix quirk in axis naming convention which used to (inadvertently) permit leading integer in name/tag of wheel or axial motor

• Tighten test cases to capture this and other oversights in original test suite

Fixes #2163

Instructions to reviewer on how to test:

1. Confirm name updates make logical sense to any reader unfamiliar with what the code does
2. Confirm You don't need to read the code in detail to understand the names - if that proves to be necessary, fail the CR
3. Confirm linters etc are happy

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 99.18%. Comparing base (b4b38f1) to head (c883930).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2164   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files         360      360           
  Lines       14296    14300    +4     
=======================================
+ Hits        14180    14184    +4     
  Misses        116      116           

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