Add multiple treatments notebook: multiple treatments training, erupt

[AlxdrPolyakov]

Problem

Describe the problem your PR is trying to solve Add multiple treatments notebook: multiple treatments training, erupt

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.

Types of changes

What types of changes does your code introduce to causaltune? Put an x in the boxes that apply

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] Description above provides context of the change
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] Unit tests for changes (not needed for documentation changes)
• [x] Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
• [x] Commits follow "How to write a good git commit message"
• [x] Relevant documentation is updated including usage instructions