Fix "source" name issues for product_store

[greenc-FNAL]

Fixes #120.

#120 reported that diagnostic output in tests was empty or malformed.

1. The malformed output was due to product_store saving a string_view to a temporary string provided at construction time.

2. The empty output had two different causes:

   1. The base() product store was not initialized with a name.
   2. At least one test (hierarchical_nodes) creates child product stores without providing a name.

• Problem 1 is fixed by ensuring that strings are passed and saved rather than string_views.
• Problem 2.1 is fixed by allowing the static member function product_store::base() to accept an optional base_name, which defaults to "Source" if not provided.
• Problem 2.2 is fixed by passing a name ("levels_to_process") to the function calls creating child product stores.

There remains a question of whether product_store::make_child() should:

1. Continue to allow for no name specification
2. Require a name specification
3. Provide a default

[greenc-FNAL]

@phlexbot format

[github-actions]

Code-formatting changes pushed (commit 1629297)

[github-actions]

No CMake formatting changes to make

[knoepfel]

Thanks, @greenc-FNAL, for the PR. Just a few comments below.

[greenc-FNAL]

@phlexbot format

[github-actions]

No CMake formatting changes to make

[github-actions]

Code-formatting changes pushed (commit a6aa7fd)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   80.22%   80.26%   +0.04%     
==========================================
  Files         115      115              
  Lines        1906     1910       +4     
  Branches      302      302              
==========================================
+ Hits         1529     1533       +4     
  Misses        247      247              
  Partials      130      130              

Flag	Coverage Δ
unittests	80.26% <100.00%> (+0.04%)	⬆️

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

Files with missing lines	Coverage Δ
phlex/core/declared_unfold.cpp	70.58% <100.00%> (ø)
phlex/core/declared_unfold.hpp	94.33% <ø> (ø)
phlex/model/product_store.cpp	100.00% <100.00%> (ø)
phlex/model/product_store.hpp	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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[greenc-FNAL]

@phlexbot format

[github-actions]

Code-formatting changes pushed (commit c30a3b9)

[github-actions]

No CMake formatting changes to make