Geo filter on a global scale

[coreation]

Currently some resources support the usage of a geo filter such as OGDWienJSON and KMLGhent, however it would be nice to have this on a higher level (just like the OSpec filter).

This can be done by adding another Filter, which inherits from AFilter (found in model/filters). The filter() function must be implemented, for examples look at the RESTFilter or the SearchFilter.

The usage of the filter comes in the RController ( GET-function ). Starting from line 109 the filters are 1 by 1 called upon, I suggest the geo filter be placed after the opensearch filter, this will already narrow down the amount of entries in the returned object, because the geo-filter will require some triangulation math (which is imho more intensive than the average OSpec filter) so the less entries, the less work the geo filter will require.