support generic reducers in aggregate_spatial (aka zonal_statistics)

[soxofaan]

aggregate_spatial/zonal_statistics in openeo-geopyspark-driver (and openeo-python-driver) assumes the provided reducer is a simple one-process reducer, and only supports: 'histogram', 'sd', 'median' or 'mean'

(also: I think it would be cleaner if we could also remove the usage of old-style "zonal_statistics" and use 1.0-style "aggregate_spatial" in more places)

[soxofaan]

also see Open-EO/openeo-python-client#144

[jdries]

Note that this is far from trivial, the current code path is based on computing histograms, and can only support statistics based on that.
It would be nice to have a use case or example of what would be useful to additionally support.

[soxofaan]

Indeed, generic aggregate_spatial reducers will be lot of work with probably not a lot of uptake at the moment.

However there are still some simple one-process reducers we could easily add (based on histograms),
e.g. from the "reducer" category listing at https://processes.openeo.org/:

• max and min
• count basic version: count valid elements (condition=null), or count all elements (condition=true)
• quantiles (but there is an additional array_element necessary to extract a scalar value from the quantiles array)

[jdries]

this one got fixed, we now also support using multiple reducers.
Fully generic is harder, so we focus on the list marked as 'reducer' in openeo-processes.