port some routines to C++

[zingale]

PR summary

This ports the hydro and diffusion timestep limiters and the thermo source to C++

PR checklist

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

[zingale]

tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2020-04-13-002/index.html

stuff with a general EOS can be a little different due to the slight diffs between the Fortran and C++ EOS.

[maxpkatz]

Looks good for CPU. You'll need to make sure lambda capture is working correctly for the CUDA build.

[zingale]

updated CPU tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2020-04-17-001/index.html