Added Ryu & Jones 3B shock tube to test rarefaction waves

examples/MHDTests/RyuJones_3B/README

+Ryu & Jones 3B 3D MHD shock tube.
+This test requires setting mu_0 = 1 and gamma = 5/3.
+We recommend running with quartic spline kernel ;
+the parameters in the .yml file are chosen to work best in that configuration.
+The fiducial set up uses stacked BCC lattices in the ICs, 
+but you can instead use glass files by running 
+
+./getGlass.sh
+
+and addapting the makeIC.py script accordingly.