more fixes for amr.subcycling_mode = None + retry

[zingale]

PR summary

PR motivation

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

[yut23]

Is this bite taken out of the secondary an issue? inputs_2d_collision.test says it sets problem.collision_separation = 1.25 to start with the stars already merging, but these don't look like very well-behaved initial conditions.

[zingale]

if you build Sedov in 2D and run as:

./Castro2d.gnu.MPI.ex inputs.2d.sph_in_cylcoords castro.change_max=1.01 amr.subcycling_mode=None castro.fixed_dt=2.e-5

then it retries after step 2 and dies in the Riemann solver

[zingale]

actually, it runs if we add castro.riemann_solver=0

[zingale]

okay... with this PR, the Sedov retries a bit at the start and then runs fine. Looking at the output, it looks reasonable (by eye). I will compare to the subcycling case in a bit.

On development, it constantly retries and then dies with too many subcycles.

So I think that these fixes are needed.

[yut23]

This number (which I'm assuming is some sort of placeholder) seems to be leaking into the state variables. Changing it to 1.e40 changes all the invalid density values, for example from -5e+65 to -5e+45.

Castro/Source/gravity/Gravity.cpp

Line 396 in 5f44c13

grad_phi_curr[level][n]->setVal(1.e50);

[zingale]

regression tests pass:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2024-04-19-001/index.html