Upgrade existing cache version to a new version

[frankenjoe]

With large databases I sometimes do the following:

1. go to the audb cache and delete all table files
2. rename version folder from previous, e.g. 1.0.0, to new version, e.g. 1.0.1
3. load new version with audb.load()

I usually do this with bug fixes to avoid having two very similar versions where anyway only one should be used in the cache.

I think it might be useful to provide a upgrade() function that converts version A to version B in a similar way. It could be implemented even smarter by only downloading the tables that actually differ between the versions.

[hagenw]

In #225 I also propose to download each table only once and store it under the version it was created/changed.