gwstrain FPE when running with true SDC

[zingale]

For true SDC (USE_TRUE_SDC = TRUE), when we finish a step and enter the sum_utils, we get an FPE in the gwstrain calculation. This is likely because the true SDC algorithm never fills the new Gravity_Type state data.

To reproduce this bug, compile bubble_convergence in debug mode, make sure that castro.sum_interval=1 is set, and run with amrex.fpe_trap_invalid=1.