Add Pressure-Energy (P-U) SPH

This MR adds Pressure Energy SPH, and makes one breaking change: renaming pressure-entropy SPH from hopkins to pressure-entropy. Users will need to configure --with-hydro=pressure-entropy rather than the --with-hydro=hopkins that they are used to.

Results from the hydro tests are already available on the wiki: https://gitlab.cosma.dur.ac.uk/swift/swiftsim/wikis/hydro-tests

It might be worth somebody having a look to see if I'm doing anything stupid performance wise, but from my perspective things have been running as fast as P-E.

Implements #197 (closed).