Fetch data if not present locally

[khaeru]

At iiasa/message_data#184 (currently private), @francescolovat notes that historical.process(1) fails on CI with:

FileNotFoundError: [Errno 2] No such file or directory: '/opt/python3/lib/python3.7/site-packages/item/data/historical/input/T000_input.csv'

• On this CI workflow, the transport-energy package is installed from PyPI, not from source.
• By design, the package does not include raw input data.
• However, the package should fall back to retrieving the data if it is not found locally.