Allow derived coord references to be lazy.

[pp-mo]

Noticed during testing for #6404, and implementing solution #6515

The old code used src.data when attaching a reference surface to a factory, but clearly src.core_data() makes a lot more sense, as a factory generally represents a "lazy" calculation anyway.

Although worth considering a separate issue, I want to test this alongside a completed #6515 + be merged into that when it's clear it doesn't break it.
Hence WIP : intend to merge into #6515 when ready

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.87%. Comparing base (90e36fe) to head (80235cf).
Report is 46 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6517      +/-   ##
==========================================
+ Coverage   89.80%   89.87%   +0.06%     
==========================================
  Files          90       90              
  Lines       23752    23908     +156     
  Branches     4418     4459      +41     
==========================================
+ Hits        21331    21487     +156     
+ Misses       1672     1669       -3     
- Partials      749      752       +3     

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