Ballet

* Add give_advice feature for FeatureAPICheck and other checks to log message on how to fix failure
* Improve logging of GFSSFAccepter and GFSSFPruner
* Improve `__str__` for DelegatingRobustTransformer and consequently consumers
* Change default log format to SIMPLE_LOG_FORMAT
* Various bug fixes and improvements

* Add CanTransformNewRowsCheck to feature API checks

* Support `None` as the transformer in a `Feature`, it will be automatically converted to an `IdentityTransformer`
* Implement `ColumnSelector`
* Update docs
* Various bug fixes and improvements

* Re-export feature engineering primitives from various libraries
* Show type annotations in docs
* Update guides
* Various bug fixes and improvements

* Make validator parameters configurable in ballet.yml file (e.g. λ_1 and λ_2 for GFSSF algorithms)
* Support dynaconf 3.x

* Accept logger names, as well as logger instances, in `ballet.util.log.enable`
* Updated docs