Resolve RAP-204 -- implements automatic downloading of skymodels

Introduces three new parset options: download_initial_skymodel, download_initial_skymodel_radius and download_initial_skymodel_server. These allow rapthor to automatically download a sky model (stored under skymodels/skymodel.txt in the working directory) out to a given radius, such that a user no longer has to specify their own sky model. By default it will download from TGSS out to a radius of 5 degrees.