fix the wdmerger case when the 2 stars are very close

[zingale]

the initialization was overlapping and doing bad things now we simply look at which model provides the largest density at a location and use that.

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

[zingale]

Here's the wdmerger 2D collision test case now:

compare to the image in #2824

[zingale]

tests: http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2024-04-20/index.html

[yut23]

The velocities need to be corrected too. I think the same condition should work.

[zingale]

velocities updated, but I want to rerun the suite on this now.