refactor into a simulation class

Author: Michael Zingale

compressible/__init__.py

@@ -3,10 +3,4 @@
 second-order (piecewise-linear), unsplit method of Colella 1990.
 
 """
-
-from initialize import *
-from preevolve import *
-from evolve import *
-from timestep import *
-from dovis import *
-
+from simulation import *