Change component sources from MOSRS mirror to Github

[Whyborn]

With the recent arrival of the UKMO to Github, we can make forks of the various components required for AM3, and retrieve the models from Github rather than the MOSRS mirror on Gadi. We will still be using the resources functionality, due to the FCM build tool.

Starting with UKCA as a test case. If that goes well, we'll do it for socrates, shumlib and gcom.