GpsProcessing : refactor out SentinelHub logic

[soxofaan]

GpsProcessing (in backend.py) has a lot of SentinelHub details woven in, with a lot of nested functions and classes. This makes it hard to maintain and write tests for. As noted in code itself:

openeo-geopyspark-driver/openeogeotrellis/backend.py

Lines 1395 to 1397 in f66a2e5

	# TODO: encapsulate this SHub stuff in a dedicated class?
	def _scheduled_sentinelhub_batch_processes(self, process_graph: dict, api_version: Union[str, None],
	job_registry: JobRegistry, user_id: str, job_id: str,

Example: as (quick)fix for #248 I added

openeo-geopyspark-driver/openeogeotrellis/backend.py

Lines 1481 to 1483 in f66a2e5

	elif isinstance(geometries, DriverVectorCube):
	# TODO: only area of covered regions instead of whole bbox area?
	return geometries.get_bounding_box_area()

but I couldn't figure out an easy way to cover that with a unit test.