Update version number, notebooks and changelog for v1.6.1 release

[w-k-jones]

• Have you followed our guidelines in CONTRIBUTING.md?
• Have you self-reviewed your code and corrected any misspellings?
• Have you written documentation that is easy to understand?
• Have you written descriptive commit messages?
• Have you added NumPy docstrings for newly added functions?
• Have you formatted your code using black?
• If you have introduced a new functionality, have you added adequate unit tests?
• Have all tests passed in your local clone?
• If you have introduced a new functionality, have you added an example notebook?
• Have you kept your pull request small and limited so that it is easy to review?
• Have the newest changes from this branch been merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.55%. Comparing base (aae14ed) to head (03fe328).
Report is 4 commits behind head on RC_v1.6.x.

Additional details and impacted files

@@            Coverage Diff             @@
##           RC_v1.6.x     #518   +/-   ##
==========================================
  Coverage      63.55%   63.55%           
==========================================
  Files             27       27           
  Lines           3844     3844           
==========================================
  Hits            2443     2443           
  Misses          1401     1401           

Flag	Coverage Δ
unittests	63.55% <100.00%> (ø)

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[github-actions]

Linting results by Pylint:

Your code has been rated at 8.31/10 (previous run: 8.31/10, +0.00)
_{The linting score is an indicator that reflects how well your code version follows Pylint’s coding standards and quality metrics with respect to the RC_v1.6.x branch.
A decrease usually indicates your new code does not fully meet style guidelines or has potential errors.}

[freemansw1]

Thanks for managing this @w-k-jones