Improve Raspberry Pi installation process

[cortadocodes]

Summary

Make the Raspberry Pi installation process simpler and more robust by reducing the number of python and system dependencies needed to run data-gateway.

Contents (#72)

Dependencies

• Remove blake3 as a development dependency and mock it in tests (note that it's still used by the cloud function)
• Add requirements-pi.txt and requirements-pi-dev.txt for installing on Raspberry Pi
• Use octue==0.29.2 to avoid h5py import error

Other

• Update installation instructions in README