External TW_STIME

[gonsie]

This PR closes the loop on how the TW_STIME API should be implemented by model developers. It does not make sense for a model-specific tw_stime to be added to ROSS core, particularly if the tw_stime object needs to be developed over time as the model is developed. With this PR, model developers can tell ROSS at configuration time where to find the implementation of tw_stime class, files which may exist outside of the ROSS repo.

I definitely need to update the TW_STIME API documentation. I’ve run local tests and there should be no impact to models which do not take advantage of this.

This replaces #168.

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If this merge represents a feature addition to ROSS, the following items must be completed before the branch will be merged:

• [ ] Document the feature on the blog (See the website Contributing guide). Include a link to your blog post in the Pull Request.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (develop@f4995ac). Click here to learn what that means. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             develop     #178   +/-   ##
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  Coverage           ?   58.26%           
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  Files              ?       33           
  Lines              ?     3563           
  Branches           ?        0           
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  Hits               ?     2076           
  Misses             ?     1487           
  Partials           ?        0           

Impacted Files	Coverage Δ
core/tw-opts.c	23.37% <0.00%> (ø)
core/gvt/mpi_allreduce.c	90.62% <100.00%> (ø)
core/tw-pe.c	96.00% <100.00%> (ø)

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