Planetary examples

Replace the broken planetary example with a simple Earth impact.

TODO: put the initial conditions at http://virgodb.cosma.dur.ac.uk/swift-webstorage/ICs/Planetary/earth_impact.hdf5 From here: http://astro.dur.ac.uk/~cklv53/earth_impact.hdf5

Fixes issue #633 (closed)

examples/GiantImpacts/GiantImpact/README.md

-An example planetary simulation of a giant impact onto the young Uranus with 
-~10^6 SPH particles, as described in Kegerreis et al. (2018), ApJ, 861, 52.
-
-This example requires the code to be configured to use the Planetary
-hydrodynamics and equation of state:
- --with-hydro=planetary --with-equation-of-state=planetary