Ballet

* Add `load_data` method with built-in caching to project API
* Fix bug in GFSSF accepter
* Always use encoded target during validation
* Various bug fixes and improvements

* Add sample analysis notebook to project template
* Add binder url/badge to project template
* Fix bug with enabling logging with multiple loggers

* Add client for easy interactive usage (`ballet.b`)
* Add binder setup to project template

* Revamp project template: update project structure, create single API via FeatureEngineeringProject, use and add support for pyinvoke, revamp build into engineer_features, support repolockr bot
* Improve ballet.project.Project: can create by ascending from given path, can create from current working directory, can resolve arbitrary project symbol, exposes project's API
* Check for and notify of new release of ballet during project update (`ballet update-project-template`)
* Add ComputedValueTransformer to ballet.eng
* Move stacklog to separate project and install it
* Add validators that {never,always} accept submissions
* Add feature API checks to ensure that the feature can fit and transform a single row
* Add feature engineering guide to documentation and significantly expand contributor guide
* Add bot installation instructions to maintainer guide
* Add type annotations throughout
* Drop support for py35, add support for py38
* Deprecate modeling code
* Various bug fixes and improvements

* Implement GFSSF validators and random validators
* Improve validators and allow validators to be configured in ballet.yml
* Improve project template
* Create ballet CLI
* Bug fixes and performance improvements

* Add project template and ballet-quickstart command
* Add project structure checks and feature API checks
* Implement multi-stage validation routine driver