Fetch only important time series with latest data

[bbengfort]

The time series that the fetch module gets should be listed in a JSON document, not as embedded Python code.

Alternatively this could also be stuck into a database.

[bbengfort]

We now have 1600+ series in the database with which we fetch. This is a lot of information and doing a complete fetch takes around 1700 seconds (which is actually pretty good if you think about it). As we update on a month to month basis, we should only update certain series and only get data for the last year.