Protect against use_post_step_regrid when not subcycling (#2639)

The castro.use_post_step_regrid parameter doesn't make sense when we're not subcycling. By the time we finish the coarse step we will already have finished all of the fine timesteps (thanks to the recent change in #2505), so there's nothing left to do.

Source/driver/Castro.cpp

@@ -216,6 +216,7 @@ Castro::read_params ()
     initialize_cpp_runparams();
 
     ParmParse pp("castro");
+    ParmParse ppa("amr");
 
     using namespace castro;
 
@@ -441,6 +442,13 @@ Castro::read_params ()
         }
     }
 
+    // Post-timestep regrids only make sense if we're subcycling.
+    std::string subcycling_mode;
+    ppa.query("subcycling_mode", subcycling_mode);
+    if (use_post_step_regrid == 1 && subcycling_mode == "None") {
+        amrex::Error("castro.use_post_step_regrid == 1 is not consistent with amr.subcycling_mode = None.");
+    }
+
 #ifdef AMREX_PARTICLES
     read_particle_params();
 #endif
@@ -545,7 +553,6 @@ Castro::read_params ()
 
    }
 
-   ParmParse ppa("amr");
    ppa.query("probin_file",probin_file);
 
     Vector<int> tilesize(AMREX_SPACEDIM);