Allow admin to specify selection strategy for the WMS dimeensions default values

Replaces pull 430:
* makes use of the GeoTools NearestVisitor (https://github.com/geotools/geotools/pull/386)
* uses a single squashed commit (we normally ask for a single commit per pull, this one just keeps the nearest visitor change aside for visibility and review, but I can squash it too before committing)