Simplify to_samples: replace OnlyAccsVarInfo block with rand(model)

[anurag-mds]

Closes #1312

Replaces the manual OnlyAccsVarInfo initialization block in to_samples with a single rand(model) call, which does the same thing, which samples from the model prior to obtain a template VarNamedTuple the same result in one line.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.69%. Comparing base (840c5d7) to head (3452131).

Additional details and impacted files

@@             Coverage Diff              @@
##           breaking    #1315      +/-   ##
============================================
- Coverage     78.70%   78.69%   -0.02%     
============================================
  Files            49       49              
  Lines          3484     3482       -2     
============================================
- Hits           2742     2740       -2     
  Misses          742      742              

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[penelopeysm]

As it turns out, looking at this more closely, the comment directly above the changed code was correct, and we can't use rand(model) because that doesn't include the results of := statements. So my issue was wrong as well. Sorry!

[anurag-mds]

Thanks for looking into it more closely, Penny! Good to know the distinction -- I learned something about := variables from this.