Ballet

* Fix bug with deepcopying `ballet.pipeline.FeatureEngineeringPipeline`

* Fix bug with deepcopying `ballet.eng.base.SubsetTransformer` ([90](https://github.com/ballet/ballet/issues/90))
* Add `ballet.drop_missing_targets` primitive

* Support missing targets in discovery and feature performance evaluation ([89](https://github.com/ballet/ballet/pull/89))
* Add `ninputs` to summary statistics in `ballet.discovery.discover`

* Improve discrete column detection in the case of many repeated values
* Add `ncontinuous` and `ndiscrete` to summary statistics in `ballet.discovery.discover`

* Defer computation of some expensive summary statistics in `ballet.discovery.discover`

* Support callable as feature input ([88](https://github.com/ballet/ballet/pull/88))