define a common dataset for a node

With the reborn OWM there started some work on the clients again, e.g.
* https://github.com/freifunk/openwifimap-api/pull/12
* https://github.com/freifunk/openwrt-packages/pull/48
* https://github.com/Freifunk-Spalter/packages/issues/159

This raises the issue of using different models of a node-dataset. technically this is no problem on the OWM-API / OWM-database, but might become a mightmare to the frontends using the nodes dataset. Having a web-statistic which will only work for data uploaded with certain client, is very likely nothing we want to have.
Example:
~~~
{"node": "some node", "latitude":52.436757800214,"longitude":13.546518087387, "height": 20, "script":"luci-app-owm"}
~~~
vs.
~~~
{"node": "some node", position: {"latitude":52.436757800214,"longitude":13.546518087387, "height": 20}, "script":"fancy-owm"}
~~~

I suggest we make a small definition of what data and a suggested format
* minimal data required
* common data that should be supplied
* optional data that can be provided