ci: Add workflow for nightly tests of notebooks

[matthewfeickert]

Description

Resolves #1077

To speed up the PR process, as the notebooks are the slowest tests by far, have the notebooks no longer be tested every PR but in a workflow that runs nightly on a CRON job and through workflow dispatch. This will ensure that any breaking changes that get through are caught within 24 hours, while speeding up CI. Workflow dispatch will also allow for the notebook tests to be run on any branch.

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

* Migrate notebook tests from main CI to notebook testing workflow to speed up PRs
   - Run on as a nightly CRON job as well as through workflow dispatch
   - Don't run on pushes or PRs

[codecov]

Codecov Report

Merging #1078 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1078   +/-   ##
=======================================
  Coverage   96.81%   96.81%           
=======================================
  Files          59       59           
  Lines        3490     3490           
  Branches      499      499           
=======================================
  Hits         3379     3379           
  Misses         68       68           
  Partials       43       43           

Flag	Coverage Δ
#unittests	96.81% <ø> (ø)

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