Rename product_store::id() to product_store::index()

[knoepfel]

This is to better express that a given product store corresponds to a given data-cell index.

N.B. There are no functionality changes with this PR; just a renaming of a member function.

[greenc-FNAL]

Review the full CodeQL report for details.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
phlex/core/declared_predicate.hpp	80.00%	0 Missing and 1 partial ⚠️
phlex/core/declared_provider.cpp	0.00%	1 Missing ⚠️
phlex/core/declared_unfold.cpp	50.00%	1 Missing ⚠️
phlex/core/declared_unfold.hpp	83.33%	0 Missing and 1 partial ⚠️
phlex/core/message.cpp	0.00%	0 Missing and 1 partial ⚠️
phlex/core/multiplexer.cpp	66.66%	1 Missing ⚠️

❌ Your project status has failed because the head coverage (75.55%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   75.59%   75.55%   -0.04%     
==========================================
  Files         126      126              
  Lines        2954     2954              
  Branches      519      519              
==========================================
- Hits         2233     2232       -1     
  Misses        500      500              
- Partials      221      222       +1     

Flag	Coverage Δ
unittests	75.55% <88.88%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
form/form_module.cpp	84.78% <100.00%> (ø)
phlex/core/declared_fold.hpp	94.82% <100.00%> (ø)
phlex/core/declared_observer.hpp	100.00% <100.00%> (ø)
phlex/core/declared_provider.hpp	100.00% <100.00%> (ø)
phlex/core/declared_transform.cpp	66.66% <100.00%> (ø)
phlex/core/declared_transform.hpp	91.48% <100.00%> (ø)
phlex/core/framework_graph.cpp	90.32% <100.00%> (ø)
phlex/core/message_sender.cpp	68.18% <100.00%> (ø)
phlex/core/store_counters.cpp	86.36% <100.00%> (-2.28%)	⬇️
phlex/model/product_store.cpp	93.33% <100.00%> (ø)
... and 7 more

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[marcpaterno]

The matches the terminology in the CDR.