quantiles support in aggregate_spatial

[soxofaan]

Reported by @przell in https://discuss.eodc.eu/t/how-to-use-quantiles-on-a-2d-data-cube-x-y/703: aggregate_spatial does not seem to support quantiles process

reproduction example:

cube = con.load_collection(
    "SENTINEL2_L2A",
    temporal_extent=["2023-09-01", "2023-09-15"],
    bands=["B02"]
)
aggregates = cube.aggregate_spatial(
    geometries=shapely.geometry.box(3, 50, 3.001, 50.001),
    reducer=lambda data: data.quantiles([0.5])[0]
)
aggregates.download("forum703-aggregates.json")

gives

OpenEoApiError: [400] BadRequest: java.lang.IllegalArgumentException: The process Constants are not expected in a reducer callback, got value 0.5. is not supported to be used in a reducer callback. (ref: r-24030792f2cb445bbb8b9a8881b5f164)

[soxofaan]

full process graph for completeness:

{
  "process_graph": {
    "loadcollection1": {
      "process_id": "load_collection",
      "arguments": {
        "bands": ["B02"],
        "id": "SENTINEL2_L2A",
        "spatial_extent": null,
        "temporal_extent": ["2023-09-01", "2023-09-15"]
      }
    },
    "aggregatespatial1": {
      "process_id": "aggregate_spatial",
      "arguments": {
        "data": {"from_node": "loadcollection1"},
        "geometries": {
          "type": "Polygon",
          "coordinates": [
            [[3.001, 50], [3.001, 50.001], [3, 50.001], [3, 50], [3.001, 50]]
          ]
        },
        "reducer": {
          "process_graph": {
            "quantiles1": {
              "process_id": "quantiles",
              "arguments": {
                "data": {"from_parameter": "data"},
                "probabilities": [0.5]
              }
            },
            "arrayelement1": {
              "process_id": "array_element",
              "arguments": {"data": {"from_node": "quantiles1"}, "index": 0},
              "result": true
            }
          }
        }
      },
      "result": true
    }
  }
}

and stack trace in logs:

  File "/opt/venv/lib64/python3.8/site-packages/openeo_driver/ProcessGraphDeserializer.py", line 1594, in apply_process
    return process_function(args=ProcessArgs(args, process_id=process_id), env=env)
  File "/opt/venv/lib64/python3.8/site-packages/openeo_driver/ProcessGraphDeserializer.py", line 1142, in aggregate_spatial
    return cube.aggregate_spatial(geometries=geoms, reducer=reduce_pg, target_dimension=target_dimension)
  File "/opt/venv/lib64/python3.8/site-packages/openeogeotrellis/geopysparkdatacube.py", line 1354, in aggregate_spatial
    visitor = GeotrellisTileProcessGraphVisitor(_builder=get_jvm().org.openeo.geotrellis.aggregate_polygon.SparkAggregateScriptBuilder()).accept_process_graph(reducer)
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 77, in accept_process_graph
    self.accept_node(graph[top_level_node])
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 88, in accept_node
    self._accept_process(process_id=pid, arguments=arguments, namespace=namespace)
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 100, in _accept_process
    self._accept_argument_dict(value)
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 118, in _accept_argument_dict
    self.accept_node(value['node'])
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 88, in accept_node
    self._accept_process(process_id=pid, arguments=arguments, namespace=namespace)
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 96, in _accept_process
    self._accept_argument_list(value)
  File "/opt/venv/lib64/python3.8/site-packages/openeo/internal/process_graph_visitor.py", line 113, in _accept_argument_list
    self.constantArrayElement(element)
  File "/opt/venv/lib64/python3.8/site-packages/openeogeotrellis/processgraphvisiting.py", line 65, in constantArrayElement
    self.builder.constantArrayElement(value)
  File "/usr/local/spark/python/lib/py4j-0.10.9.7-src.zip/py4j/java_gateway.py", line 1322, in __call__
    return_value = get_return_value(
  File "/usr/local/spark/python/lib/py4j-0.10.9.7-src.zip/py4j/protocol.py", line 326, in get_return_value
    raise Py4JJavaError(
py4j.protocol.Py4JJavaError: An error occurred while calling o517246.constantArrayElement.
: java.lang.IllegalArgumentException: The process Constants are not expected in a reducer callback, got value 0.5. is not supported to be used in a reducer callback.
	at org.openeo.geotrellis.aggregate_polygon.SparkAggregateScriptBuilder.badReducer(SparkAggregateScriptBuilder.scala:92)
	at org.openeo.geotrellis.aggregate_polygon.SparkAggregateScriptBuilder.constantArrayElement(SparkAggregateScriptBuilder.scala:79)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)