Replace do_real_eos = 0 with an actual EOS

[maxpkatz]

PR summary

This PR adds an EOS, rad_power_law, which replaces the functionality previously provided by radiation.do_real_eos = 0. The EOS defines c_v as before, and can only be called in (rho, T) and (rho, e) modes. It does not define hydrodynamic variables like pressure, so it cannot be used in conjunction with castro.do_hydro = 1. This sets us up for merging the radiation and non-radiation EOS interfaces.

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 are here:

http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2020-01-02-003/index.html

note that RadSuOlsen crashes with negative rho e

[zingale]

answers change a lot in the current RadSuOlson test

http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2020-01-02-004/rad-SuOlson.html

[maxpkatz]

I looked at the old code. We were previously setting T and e to 0 in ca_initdata. What was happening in the computeTemp call after ca_initdata is that the old EOS's approach of flooring the data by adding 1e-50 meant that it was treating rho*e as 1.e-50, and then calculating T from there. So the original data was completely arbitrarily determined by that floor.

Since the initial values don't really matter, I've set these to be the same as they effectively were before (1.e-50) so that results don't change much.