Scala version of process graph visitor

[jdries]

In Python, we have this class:

https://github.com/Open-EO/openeo-geopyspark-driver/blob/c7c24656ec83174466a1a9c038b59811b3a297bc/openeogeotrellis/processgraphvisiting.py#L11

It walks over a process graph, and invokes callbacks on a scala object.
We don't have an equivalent in scala, and as a consequence always have to write out callback invocation manually, which is very tedious.

Can we port this class to scala, preferably preserving the separation between an abstract base class and an implementation that invokes callbacks on OpenEOProcessScriptBuilder?

The hope is that we can then store process graphs as json in the scala repo, and easily set up test scenario's based on that json.
Maybe we can even define scenario's entirely in json...