Resolve RAP-304

Aims to resolve the clash between automatic skymodel download and restarting a run. The overwrite argument of miscellaneous.download_skymodel is now exposed through the parset option download_overwrite_skymodel and the following logic is applied:

1. If input_skymodel is not set in the parset, and download_initial_skymodel is set to True, a skymodel will be downloaded and apparent_skymodel will be set to None, such that the latter is not accidentally used later on.
2. If input_skymodel is set in the parset and download_initial_skymodel is set to True, the user-supplied model takes priority and one will not be downloaded
3. If situation 2, but download_overwrite_skymodel is also set to True then the user-specified skymodel is overwritten.
4. If input_skymodel is set, but does not exist, and no download is requested, it will warn the user and exit.