feat: Add toy sampling

[lukasheinrich]

Description

toys

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add toy sampling capability
* Remove projection methods from constraint
* Add generalized tensorviewer (expect to merge w/ ParamViewer)
* Introduce make_pdf method to construct pdf with frozen parameters, evaluate with log_prob
* Add Poisson and Normal classes to Numpy backend with same API as PyTorch/Tensorflow

[coveralls]

Coverage increased (+0.1%) to 93.586% when pulling a86e8fc on add_probability_module_withjoint_withextended_pdf_api_withtoys into b8f524a on master.

[matthewfeickert]

I migrated docstrings from PR #551, but I'll come back to this later.

[lgtm-com]

This pull request introduces 1 alert when merging 316a936 into 3206e97 - view on LGTM.com

new alerts:

• 1 for Module is imported with 'import' and 'import from'

[lgtm-com]

This pull request introduces 2 alerts when merging 5af5af1 into 3206e97 - view on LGTM.com

new alerts:

• 2 for Comparison using is when operands support __eq__

[matthewfeickert]

@lukasheinrich Thank you for this awesome PR and for all the work and PRs that you did leading up to this. Everything LGTM and once the CI passes I'll happily merge this in.