Added new local maxim_db lookup and accessor for city, for the new API, whi…

[pgruener]

There's also an open issue for this topic #1432

My changes are documented (in README_API_GUIDE.md) and describe how they allow to deal with the updated maximdb download API.

I didn't have enough time, for making it better but in my opinion it's better than a doc, which describes an experimental followup script, which simply doesn't work. (As it is currently - since a year).

I inherited when possible, and duplicated some code (where inheritance wasnt possible).

So the available code will work as before (but the creation of the migration, which is just outdated; and people who already used the old state, already migrated their databases, so it seemed useless to keep the state).

However I hope this helps anyone, who was looking for the same way as I did.

[richjdsmith]

This is still breaking. Your PR looks to work and ran when tested locally. Not sure how to proceed here. @alexreisner , thoughts?