Use many DBs together

[manuelma]

At the moment, using_spinedb attempts to clear whatever convenience functions have been created so far. So if the user modifies their DB, further calls to using_spinedb should reflect the new contents of the DB. Some users have reported that some artifacts from older calls remain, but I haven't been able to reproduce that.

We could be more aggressive in clearing the older convenience functions by assigning nothing to them.

But also, this should be optional. In case the user wants to use multiple DBs together, that should also be possible.

[manuelma]

Fixed by c0a5f73