Create a versioned API #518
Description
#491 introduces an api submodule, to be used by applications to import the required symbols from Fab, independent for the actual location of the files inside of Fab (allowing easily to refactor files in Fab without affecting existing applications).
@MatthewHambley suggested that Fab should have versioned API files, e.g. a submodule v1 from which to import a certain consistent set of symbols. If changes happen later, a v2 submodule can offer a new API, and the old one can be deprecated.
Since we couldn't really agree on (or better, I don't really understand) the best way to implement the versioned file, and to unblock #491, I opened this as a separate issue to be handled by a UKMO.