Fix segfault in deprecated newPhase factory function

[speth]

The shared_ptr goes out of scope at the end of the function, deleting the held ThermoPhase right as it's returned.

Erroneous behavior introduced in f3e840dcbe (#1448).

Changes proposed in this pull request

• Replace the shared_ptr-based implementation with one that uses a unique_ptr (which can be released without destroying the underlying object)
• Add test coverage for this method

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

Merging #1600 (9b55c1a) into main (8e2ab16) will increase coverage by 0.00%. The diff coverage is 58.82%.

@@           Coverage Diff           @@
##             main    #1600   +/-   ##
=======================================
  Coverage   70.59%   70.59%           
=======================================
  Files         379      379           
  Lines       59168    59185   +17     
  Branches    21257    21270   +13     
=======================================
+ Hits        41768    41781   +13     
- Misses      14324    14325    +1     
- Partials     3076     3079    +3     

Files Changed	Coverage Δ
src/thermo/ThermoFactory.cpp	74.30% <58.82%> (+0.15%)	:arrow_up:

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