[WIP] Add Pressure-Energy with Morris and Monaghan artificial viscosity

This includes a new scheme, pressure-energy-monaghan that includes a variable artificial viscosity.

More information is available here: http://adsabs.harvard.edu/abs/1997JCoPh.136...41M

This still needs documenting fully, but I think it is probably a good idea to merge this sooner rather than later to avoid divergence.

Note that we actually update the artificial viscosity half a time-step too "late" -- this does not seem to have a significant effect.

This passes all of the current hydro tests, and shows a significant improvement, particularly with respect to the width of the distributions.

See attached the SedovBlast_3D.

One minor issue is that this makes performance on the Evrard collapse marginally worse; the distribution is moved around a bit.