Python-terrier

This release includes:
- [IR Datasets](https://ir-datasets.com) support (#90, 100) - thanks to Sean MacAvaney, University of Glasgow
- faster indexing via multi-threading and fifos (92) - thanks to Sean MacAvaney, University of Glasgow
- Easier ways of operating on text of documents (88)
- Documentation improvements - thanks to Iadh Ounis, University of Glasgow
- ensuring Precisioncutoff metrics are reported (89) - thanks to Xiao Wang, University of Glasgow
- rounding of pt.Experiment values - suggestion by Iadh Ounis, University of Glasgow

This PyTerrier release contains significant updates from the last public release in March 2020. It is intended to serve as a stable release for PyTerrier, while further improvements are merged in a new stable release.