MLJBase v1.6.0

Diff since v1.5.0

• (enhancment) Arrange that pipelines support transformers that need a target variable for training (#984)

Merged pull requests:

• Docstring patches (#983) (@abhro)
• Add support in pipelines for Unsupervised models for which target_in_fit is true (#984) (@ablaom)
• Fix default_logger docstring (#986) (@ablaom)
• For a 1.6 release (#987) (@ablaom)