Env check

[bnmajor]

Adds an environment check notebook that is designed to check the environment that the user is running in to make sure that all required dependencies and extensions are correctly installed. Ideally run first before any other notebooks to prevent common issues around dependencies and extension loading.

[review-notebook-app[bot]]

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[bnmajor]

Some additional updates:

• Allow the requirements list to include whether or not it should use the pre-release. Expects the following format:

  "{package_name}{(==|>=)version_number}{ --pre}"

  Where the package name is required but the version is optional, as is the --pre flag (expected at the end if included)

• Warn users that pip occasionally will raise errors when it is unable to resolve dependency conflicts. This can usually be safely ignored but there are now links to instructions on how to create a new, clean environment (either virtual env or conda) as well as a link to opening an issue if there are actual issues running notebooks in the repo after using the env check notebook.
• Prevent a bug that could possibly force the notebook to refresh non-stop