add sklearn dictionary support get_param and version update

[ryan-wolbeck]

This pull request adds sklearn dictionary compatibility and the gamma distribution to ngboost 0.4.0.

Sklearn dictionary compatibility allows users to use the ngboost library with the same syntax and functionality as other sklearn compatible libraries. The gamma distribution can be used to fit regression and classification models.

This pull request includes the following changes:

• Added sklearn dictionary compatibility (re-created from #284)
• Bump version to 0.40 (to release package with updates here and the gamma dist)
• add release notes md file
• allow cli access to release notes