duplicate save_results after datacube_from_flat_graph

[soxofaan]

when loading a process graph with a save_result node through datacube_from_flat_graph, you get a DataCube object, instead of the new SaveResult object.
If you execute_batch this, you get an additional save_result

related to #720 #402 #725

[soxofaan]

e.g.

import openeo
from unittest import mock

connection = openeo.connect("openeo.vito.be")

pg = {
    "lc": {"process_id": "load_collection", "arguments": {}},
    "sr": {
        "process_id": "save_result", 
        "arguments": {"data": {"from_node": "lc"}},
        "result": True,
    },
}

cube = connection.datacube_from_flat_graph(pg)

with mock.patch(
    "openeo.rest.connection.Connection.create_job", 
    new=lambda *args, **kwargs: print((args, kwargs))
):
    cube.create_job()

((<Connection to 'https://openeo.vito.be/openeo/1.2/' with NullAuth>,), {'process_graph': {'loadcollection1': {'process_id': 'load_collection', 'arguments': {}}, 'saveresult1': {'process_id': 'save_result', 'arguments': {'data': {'from_node': 'loadcollection1'}}}, 'saveresult2': {'process_id': 'save_result', 'arguments': {'data': {'from_node': 'saveresult1'}, 'format': 'GTiff', 'options': {}}, 'result': True}}, 'title': None, 'description': None, 'plan': None, 'budget': None, 'validate': None, 'additional': None, 'job_options': None, 'log_level': None})

[soxofaan]

quickfixed bug with 9b1e6a4

remaining work is for #733