JP-3618: Add intermediate LRS slit wcs reference frame

[drlaw1558]

Resolves JP-3618 by adding an intermediate alpha-beta slit reference frame in the LRS distortion transform between detector pixels and v2v3.

DO NOT MERGE- requires further testing.

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• All comments are resolved
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.54%. Comparing base (17e0cd9) to head (746fad2).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8475      +/-   ##
==========================================
+ Coverage   56.38%   56.54%   +0.15%     
==========================================
  Files         387      387              
  Lines       38716    38869     +153     
==========================================
+ Hits        21830    21977     +147     
- Misses      16886    16892       +6     

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