add doctest harness and update CI

[gknapp1]

While we had an integration/smoke test in the CI by running the tutorial case, there are no unit tests for AdditiveFOAM. This adds a doctest framework for unit test.

I chose doctest because it is lightweight enough to not add much of a burden on compiling and can be compiled easily with wmake. This does mean that AdditiveFOAM would vendor (distribute) the doctest header and associated MIT License file to be compatible with the doctest license, which is common practice.

New features:

• doctest header and initial framework to provide a place to add unit tests
• basic unit test of the segment functionality to demonstrate
• update CI to use a OpenFOAM-13 container (third-party, since openfoam.org does not distribute an official OpenFOAM-13 container)
• update CI to include building and running the unit tests

In draft until:

• implementation of additional unit tests
• update once WIP V2.0 refinement model #92 is merged (though possibly will merge this first, then update other PR)