loading projection definitions on demand

[ThomasHalwax]

OpenLayers uses proj4js to re-project sources to the current view. Since we can't include every projection definition we should provide an easy way to do so.

epsg.io provides frequently used projection definitions. ODIN should

• [x] only include a minimal set of projection definitions
• [x] detect if the projection definition for a given source is missing
• [ ] download the projection definition form epsg.io if allowed by the user
• [ ] persist the downloaded definition an a (JSON) file