Draft: Public API to set caldb_root (option 1)

The one where it's attached to the CalCatAPIClient and every SingleConstant object created from that.

The upside is that you could set different versions with different clients, for production & test CalCat. But it's more complex - there are 3 places the path could come from when you load a constant instead of 2 - and attaching the path to the API client which doesn't deal with the filesystem at all feels a bit ugly.