Add P-U with M&M Switch

This is the version of Pressure-Energy SPH that includes the Morris & Monaghan (1997) variable artificial viscosity with a Balsara switch.

This should not contain too many changes in the central code. The hydro test suite is currently running but we see marginally improved convergence, see below for the 3D Gresho Vortex.

With variable AV: Without:

Morris, J. P., & Monaghan, J. J. (1997). A Switch to Reduce SPH Viscosity. Journal of Computational Physics, 136(1), 41–50. http://doi.org/10.1006/jcph.1997.5690

examples/KelvinHelmholtz_2D/makeMovie.py

@@ -91,6 +91,7 @@ if __name__ == "__main__":
 
     # Creation of first frame
     fig, ax = plt.subplots(1, 1, figsize=(1, 1), frameon=False)
+    ax.axis("off")  # Remove annoying black frame.
 
     data_x, data_y, density = load_and_extract("kelvinhelmholtz_0000.hdf5")