Cg Hub

Combine cg hub downloading, query and metadata parsing into one place under gmt cg-hub.
This seems perhaps outside of the scope, but this definitely shouldn't be hardcoded in here. Its also world readable which keyfiles definitely shouldn't be.

It seems like each user should have their own and have to supply it in order to run the command. A path could be read from the CLI, an ENV variable or similar. Either way, we definitely shouldn't be sharing one key between everyone who runs this command.