Big update for planetary stuff

Lots of minor and some major updates and tweaks, including bug fixes for particles running out of neighbours and non-periodic boundaries, some of which are crude and temporary.

Planetary simulations are now running successfully!

Also removed the examples since they need serious updating, to be replaced with functional versions soon.

examples/MoonFormingImpact/README.md

-Canonical Moon-Forming Giant Impact
-===================================
-
-NOTE: This doesn't really work because the EOS are different to Canup (2004) so
-the impactor just glances then flies away!
-
-A version of the canonical moon-forming giant impact of Theia onto the early
-Earth (Canup 2004; Barr 2016). Both bodies are here made of a (Tillotson) iron
-core and granite mantle. Only ~10,000 particles are used for a quick and crude
-simulation.
-
-Setup
------
-
-In `swiftsim/`:
-
-`$ ./configure --with-hydro=minimal-multi-mat --with-equation-of-state=planetary`
-`$ make`
-
-In `swiftsim/examples/MoonFormingImpact/`:
-
-`$ ./get_init_cond.sh`
-
-Run
----
-
-`$ ./run.sh`
-
-Output
-------
-
-`$ python plot.py`
-`$ mplayer anim.mpg`
-