swap the order of Poisson and the first burn for Strang

[zingale]

it should not matter which comes first, since reactions don't change density, but this swap will allow us to precompute the shock flag with sources before the first burn.

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