Issues from 0.0.1 after first run

• Output path is wrongly set in notebooks 0 2ec7a8cf
• Typo in file format of run_config in Using_notebooks.ipynb
• Wrong import of test_utils in Using_notebooks.ipynb
• Missing import of hdf5plugin in juno/__init__.py
• Folders of output path should be created if not existing
• Output file writing should be tested before launching the computation
• Add clearer errors messages in validate_config.py