don't compute hybrid momentum sources unless needed

[zingale]

if we compile with USE_HYBRID_MOMENTUM=TRUE but run with castro.hybrid_hydro=0, we were still computing the hybrid_sources. These are not needed.

This logic now mirrors the rotation logic.

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]

closes #2442

[zingale]

it occurs to me now that we might still want the sources though, even if we are not using the hybrid momentum to replace the linear momentum, since we still have the hybrid momentum components in the StateData / plotfiles.

So maybe the alternative is to just disable USE_HYBRID_MOMENTUM in wdmerger by default, since we are not using it.

[zingale]

closing this -- I think the current behavior is correct. Because otherwise, the state variables for the hybrid momentum are not correct, even though they are never coupled into the system.