Pyterrier-caching

What's Changed
* Improvements to DenseScorerCache by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/12
* Package Modernization by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/13
* replace artifact_builder with pyterrier-alpha's version by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/14


**Full Changelog**: https://github.com/seanmacavaney/pyterrier-caching/compare/v0.2.0...v0.2.1

What's Changed
* examples by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/1
* RetrieverCache by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/2
* Add len() to IndexerCache by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/3
* Artifact API by seanmacavaney in https://github.com/seanmacavaney/pyterrier-caching/pull/4

**Full Changelog**: https://github.com/seanmacavaney/pyterrier-caching/compare/v0.0.1...v0.1.0