Clean up spurious background-particle halos for Zoom runs

[james-trayford]

In zoom runs, VR can produce spurious halos dominated by background particles that we don't want to include in the plots.

I'm looking to implement a global mask to clean the catalogue of these objects such that halos cannot have > 10% contribution to their M200 by background particles

[MatthieuSchaller]

See also #66.