spatial "proximity" queries

opened by @tyrasd

Feedback from today's workshop: spatial proximity queries are a very important topic for geographers. An example query would be: calculate all benches that are inside of a park and in the vicinity of a tree and have an ice cream shop nearby :ice_cream:

//cc @franz-benjamin @redfrexx

**!!! please consider the detailed comments: https://gitlab.gistools.geog.uni-heidelberg.de/giscience/big-data/ohsome/oshdb/issues/104 !!!**

The following steps need to be taken, to merge this:
- [ ] More test-cases (e.g. a mapping of OSHDBSnapshot-> XYZ before the match() would result in an Exception and needs to be asserted)
- [ ] reduce number of introduced functions: we propose one function e.g. ```.spatialJoin(...)``` that takes the type of join as an argument (instead of one method per type)
- [ ] clean up code
- [ ] there is a lot of commented out code
- [ ] assure the new code meets the quality standards set out in jenkins
- [ ] Implementation for SnapshotView has priority. A solution for ContributionView should be thought of (there is one in the code which needs to be reviewed in detail)
- [ ] possible other open questions should be discussed in a personal meeting
- [ ] this issue needs an assignee and a timestamp (milestone)
- [ ] "harden" the API against potential misuse (e.g. returning a non matching mapreducer in the spatial relation callback functions); or if possible: change the API to not allow such cases