Add new way to place multiple GameObjects in pre-defined locations

[kb173]

When creating this new type of GameObject at a certain position, we don't just create a Feature at that position. Rather, we "activate" pre-set features from a GIS layer around this GameObject's position. This way, we can e.g. have pre-defined possible wind turbine locations, and creating a GameObject representing an entire "wind park" activates a certain number of those.

Alternatively (perhaps better design?), we could have a layer with possible locations and, instead of "activating" them, we place individual Wind Turbine features

It would probably be best to use 3 GIS layers for this:

1. Game Object Group Centers
2. Potential GameObject locations
3. Actual GameObjects

So creating a GameObject in 1 queries locations from 2 and creates new features based on those in 3.

[MathiasBaumgartinger]

Implemented via d2e6359, 421b9ca, 177fedd

[MathiasBaumgartinger]

We need to add a min and max cluster size attribute and prevent empty instances. Empty instances can happen if the cluster size is too big for the given radius or bigger than the available predefined features.