Option to stitch results of partitioned jobs

[soxofaan]

This has been suggested before and also came up in https://discuss.eodc.eu/t/job-splitting/467/7 :

When using partitioned jobs (aka managed job splitting, the user might prefer a single result file instead of separate files for each sub job

[jdries]

Note that openEO may already allow this by:

• load_result of the job that generated multiple results
• save_result to geotiff (default options should produce single tiff)