more logical arg/var names in sync_group; update read_group_file

The whole local-vs-remote terminology was hard to parse. "Source"
and "destination" are much easier to understand.
read_group_file() now gets repo info from new store file format.
adapt commands.sync_group to use new inventory OOP code

Instead of loading a groupfile, load an Organization object,
choose a Store, and sync the collections listed therein.