the 4th order solvers are only guaranteed to work with square cells (#340)

this asserts that

Author: zingale

pyro/compressible_fv4/simulation.py

@@ -1,3 +1,5 @@
+import numpy as np
+
 import pyro.compressible_fv4.fluxes as flx
 from pyro import compressible_rk
 from pyro.compressible import get_external_sources, get_sponge_factor
@@ -70,6 +72,9 @@ def preevolve(self):
         this solver assumes that the initialization was done to
         cell-centers and converts it to cell-averages."""
 
+        # this should only work for dx == dy
+        assert np.abs(self.cc_data.grid.dx - self.cc_data.grid.dy) < 1.e-12 * self.cc_data.grid.dx, "grid cells need to be square"
+
         # we just initialized cell-centers, but we need to store averages
         for var in self.cc_data.names:
             self.cc_data.from_centers(var)