Decouple DataLayer information from JSON representation

[frcroth]

Currently, datalayers are parsed and written via JSON macro, which is simple, but does not allow for great flexibility. E.g. when adding new properties, these have to options, so that existing datasource-properties.json can still be read.

Therfore, the JSON formatting should be done manually (cmp. https://stackoverflow.com/questions/45496635/play-framework-json-reads-from-empty-string-to-empty-list)

This could also allow a full backwards-compatible renaming of scale to voxelSize