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)

merged

mentioned in commit 46b99548

I have also copied the ICs. Let me know whether it all works.

Thanks, yeah looks good :)