swap the order of Poisson and the first burn for Strang

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.

Source/sources/Castro_sources.cpp

@@ -529,6 +529,15 @@ Castro::pre_advance_operators (Real time, Real dt)
 
     advance_status status {};
 
+    // If we are using gravity, solve for the potential and
+    // gravitational field.  note: since reactions don't change
+    // density, we can do this before or after the burn.
+
+#ifdef GRAVITY
+    construct_old_gravity(time);
+#endif
+
+
     // If we are Strang splitting the reactions, do the old-time contribution now.
 
 #ifndef TRUE_SDC
@@ -541,11 +550,6 @@ Castro::pre_advance_operators (Real time, Real dt)
 #endif
 #endif
 
-    // If we are using gravity, solve for the potential and gravitational field.
-
-#ifdef GRAVITY
-    construct_old_gravity(time);
-#endif
 
     // Initialize the new-time data. This copy needs to come after all Strang-split operators.