v3.4.0: parallel uploads

upload_files() sends files up to 16 MB four at a time (concurrency=, default 4, like the web app; concurrency=1 for the old one-after-another). Larger files follow in 16 MB chunks. A file that keeps failing stops the rest and raises; the dataset is not completed. Progress callbacks are never called twice at once.

Install: pip install git+https://github.com/SmartForest-no/forestsens-python.git@v3.4.0