Data store source should be specified in model config file

[alexdewar]

As the version of the data files used is a crucial part of the model run, I think really the path to the data source should be in the config file rather than specified as a command-line argument (or better still, as a URL: #364).

Do you agree, @jamesturner246?

[jamesturner246]

Sounds sensible! 👍