Policy reasoner nice-to-haves

[Lut99]

This issue tracks some features related to the policy reasoner that are nice-to-have. Or at least, which won't be our immediate focus.

Things left to do are:

• Add metadata to container definitions #66
• Add metadata to data definitions #65
• Add proper user identification/authorisations
  • This is clearly a much bigger project, but necessary for proper functionality with the reasoner.
• Add options to sign workflows / metadata for verification