cluster diagnostics and post-processing (before reassignment)

[Juan-Mateos]

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Checklist:

• [x] 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
• [ ] 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)
• [x] I have documented the code
  • [ ] Major functions have docstrings
  • [ ] Appropriate information has been added to READMEs
• [x] I have explained the feature in this PR or (better) in output/reports/
• [x] I have requested a code review

[Juan-Mateos]

This PR includes...

• A new script industrial_taxonomy/pipeline/text_clusters/cluster_diagnostics.py with a flow FilterClusters that...
  • calculates sic and text-sector level silhouette scores
  • filters text sectors in the bottom quartile of homogeneity (the user can change the threshold)
• New functions in industrial_taxonomy/pipeline/text_clusters/utils.py for the script above
• A modified text_getter in industrial_taxonomy/getters/text_sectors.py that only returns company assignments in non-filtered sectors.
• Instructions to run the flow in industrial_taxonomy/pipeline/README.md