Set up testing for C++ Delta Eddington solver

[mattldawson]

Set up Google Test for C++ unit and integration testing. Add regression test to existing Fortran tests that compares the Fortran and C++ implementation of the Delta Eddington solver for a given set of conditions.

Acceptance Criteria

• When the Fortran tests are run, the Fortran-based Delta Eddington solver is used to solve for the radiation field for a given set of conditions. The C++ Delta Eddington solver is used to do the same. The results are compared and the test only passes if all radiation field values are within a tolerance.
• Google Test runs a simple unit test of the stubbed C++ Delta Eddington solver in the GH Action tests

Ideas

• Google Test could be set up similarly to how it is done in MICM. See here: https://github.com/NCAR/micm/blob/ada3263ceac0f7540743b3381cfda83a8c35c8ae/cmake/dependencies.cmake#L73-L94 and here: https://github.com/NCAR/micm/blob/ada3263ceac0f7540743b3381cfda83a8c35c8ae/cmake/test_util.cmake#L22