1000 tools paper

Changes following the first round of revisions:

• Submit the repository to Zenodo and document in README.md
• Add the preprint reference to README.md and add CITATION.cff
• Update some packages used for analysis and add a few new ones
• Update the database version used for analysis to 2021-09-26
• Revise the models for tools
• Add model fits to model plots
• Add new plots showing the delay in adding tools to the database, abstract word clouds and correlations between metrics
• Add PDF figure output (and adjust scaling for this)
• Other minor updates to figures

Full Changelog: https://github.com/scRNA-tools/1000-tools-paper/compare/20210813...20211008