make the RT problem initial conditions symmetric (#216)

this reduces roundoff

Author: zingale

pyro/compressible/problems/rt.py

@@ -59,8 +59,9 @@ def init_data(my_data, rp):
 
         j += 1
 
-    ymom[:, :] = amp*np.cos(2.0*np.pi*myg.x2d/(myg.xmax-myg.xmin))*np.exp(-(myg.y2d-ycenter)**2/sigma**2)
-
+    L = myg.xmax-myg.xmin
+    ymom[:, :] = amp * 0.5 * (np.cos(2.0*np.pi*myg.x2d/L) +
+                              np.cos(2.0*np.pi*(L - myg.x2d)/L)) * np.exp(-(myg.y2d-ycenter)**2/sigma**2)
     ymom *= dens
 
     # set the energy (P = cs2*dens)