Add accessor functions, multi-h5 modeling

[mattcieslak]

This is a massive refactor based on 3/26/2026 morning hackathon.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.80992% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.51%. Comparing base (14ab04f) to head (24ee477).

Files with missing lines	Patch %	Lines
R/merge.R	71.69%	30 Missing ⚠️
R/analyse-helpers.R	91.83%	12 Missing ⚠️
R/ModelArray_S4Methods.R	88.76%	10 Missing ⚠️
R/ModelArray_Constructor.R	66.66%	5 Missing ⚠️
R/utils.R	95.83%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
+ Coverage   82.68%   89.51%   +6.83%     
==========================================
  Files           4        6       +2     
  Lines        1854     1717     -137     
==========================================
+ Hits         1533     1537       +4     
+ Misses        321      180     -141     

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