Add releases service to get layer releases timestamps

In RE2, we had the zeitreihen service [1] to get 'release years' of the zeitreihen layer for a given zoom level and a given location. This PR adds a similar service in RE3. It's a little more generic than in RE2 and can be used on any layer (of course, AFAIK only zeitreihen has release years).

This is to be used in the new per-year prints currently under dev by @procrastinatio . As discussed, this new service will be used in front-end to get the desired timestamps for printing, which are then send to the print service.

@cedricmoullet As you are the author of the zeitreihen service of re2, can you please review by taking into account the following:
- This new service doesn't use meta models [2]. In my opinion, we have everything we need in the existing zeitreihen models. Or were there specifics in the meta models that I'm not aware of?
- Instead of a point and a tolerance, this new service here takes a mapextend (meters) and a imagedisplay (pixels) to calculate the resolution/scale for the correct model. The idea is to get all releases for the selected rectangle in the print service

[1] https://svn.prod.bgdi.ch/viewvc/mf-chsdi/trunk/chsdi/chsdi/controllers/zeitreihen.py?view=markup
[2] https://svn.prod.bgdi.ch/viewvc/mf-chsdi/trunk/chsdi/chsdi/model/vector/zeitreihen.py?view=markup
That's true. But the client can decide what he wants to send. He can send a very small rectangle as well and 'simulate' old point behaviour. It's extending existing behaviour.

As discussed with Marc, I can adapt the service to only use a point/tolerance approach (as for identify request), but I would still need parameters to be able to calculate resolution on the server side to pick correct model.