Compute the shock flag once -- at the start of the timestep

[zingale]

PR summary

This changes the way the shock flag is computed for disabling burning in shocks. Now we compute it at the top of the timestep, before the first Strang burn, using the time-level n state and sources. There is some redundancy here (we need to compute the old sources before the burn now), and there is a FillPatch needed. But this should be minimal and only in effect if we compile with SHOCK_VAR.

Note: this does not change how we compute the shock flag for hydro at the moment when doing the hybrid Riemann solve (although we can easily change that in a future PR).

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]

depends on #2726 fixes #2724

[zingale]

this is ready for review now