Improve filenames for .csv export

[P3nny]

.csv filenames should look like this:

europawahl_bundesländer_2014.csv
{{merkmal}}{{region_name}}{{year}}.csv

[sjockers]

This has been implemented, but file names are not perfect yet. Before we continue to work on this, we should define a naming scheme, probably using IDs or slugs. Problems I see:

• Measures often have quite long and complicated names – how do we turn them into slugs? Should we use IDs instead?
• Region names are not unique. Should we use AGS instead? Also, we need different names for parent and direct region queries (id=01 vs parent=01).
• Measures are not unique. Should we add info about statistics, too? Should we use slugs or IDs?

[crijke]

Ideen:
13-Berlin-NUTS1-12611-BEV003.csv. / ALL-NUTS1-12611-BEV003.csv