38 metaflow extensions refactor

[bishax]

Closes #38

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Checklist:

• [ ] ~I have refactored my code out from notebooks/~
• [x] I have run flake8 and addressed any linter erors
• [x] I have checked the code runs
• [x] I have tested the code
• [x] I have run pre-commit and addressed any issues not automatically fixed
• [x] I have rebased onto dev (or merged any new changes from dev)
• [ ] ~I have documented the code~
  • [ ] ~Major functions have docstrings~
  • [ ] ~Appropriate information has been added to READMEs~
• [ ] ~I have explained the feature in this PR or (better) in output/reports/~
• [ ] I have requested a code review