enrich graphml files for download

[sennierer]

Currently the graphml file that can be downloaded offers not much metadata. It would be great to have at least some parameters such as date of birth, longitudes and latitudes etc. in the downloaded file. I guess we can either add that metadata directly to the network json or send the ids of nodes and edges in the network to an endpoint that (re)generates the graphml for download.

[stefanprobst]

the exported graphml now includes all scalar node/edge properties. properties with object/array values are ignored -- it is up to the library consumer to flatten those attributes.