allow multiple outputs for inference results

[jdegerickx]

When saving an intermediate result in openeo, one needs to specify the "filename_prefix" option in the "save_result" function.
This was not done before, so the asset was overwritten by the final result.

I tested this for cropland inference and it works now, successfully generating 2 assets.

Instead of downloading the assets during the "execute_batch" call (which only supports download of a single asset), I have introduced a few lines which download all assets belonging to a certain job, including the metadata json file.

I changed the nature of InferenceResults, to be able to cope with multiple outputs for a single inference run.
To do so, I renamed WorldCerealProduct to WorldCerealProductType (because this was actually a product type) and created a new data class WorldCerealProduct which now holds the URL, local path and product type of an actual product (raster file).

All changes were tested for the cropland inference case.
I was not yet able to test inference for crop type, as the job failed due to another reason...

[kvantricht]

Nice evolution! But I added quite a few comments to think about and we can discuss on Monday. File naming consistency is something we have to think about carefully.

[kvantricht]

good to go!