Convert hbflx3 to C++

[maxpkatz]

PR summary

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[zingale]

tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-10-01/index.html
there are some FPEs

[maxpkatz]

tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-10-01/index.html there are some FPEs

I had an indexing error which is now fixed, can you try again?

[maxpkatz]

I've synchronized 3D with 1D and 2D. The 3D case was making some simplifying assumptions; the more general expression in 1D and 2D are now what we use, and they result in the 3D expression because r0 = 1 for 3D Cartesian.

[zingale]

updated tests:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-10-07-004/index.html

[zingale]

this works as expected in 3D?

[maxpkatz]

This PR now condenses all six faces of the grid into a single code block, and instead of using the vector of metric terms computed in a separate step, it now evaluates those metric terms inline.

[zingale]

all radiation tests pass:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2023-10-09-001/index.html