Add option to define different n_min_threshold dependent on thresholds

[JuliaKukulies]

This PR solves #18 by allowing users to input n_min_threshold (the minimum number of grid cells that must be above/below the set threshold(s)) as dict or list. That way, different n_min_threshold values are used for different threshold values in the feature detection.

The code changes encompass:

• Changes to the function feature_detection_multithreshold_timestep() because here, individual threshold values are selected and passed to feature_detection_threshold()
• Input validation to make sure that each threshold is assigned to one n_min_threshold. This means that the number of elements of the given dict or list has to be the same as the number of given thresholds. It is also made sure that these values are sorted accordingly when threshold values are sorted. When n_min_threshold is given as dict, the keys must correspond to the threshold values. When n_min_threshold is given as list, it is assumed that each n_min_threshold belongs to the value in threshold at the same position.
• Additional test parameters in test_feature_detection.py to test the new feature
• Modifications to the existing notebook on the input parameter n_min_threshold to show an example on our RTD page: see Different n_min_threshold for different threshold valuesunder Feature Detection Parameter Examples https://tobac--208.org.readthedocs.build/en/208/

• 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

Base: 41.01% // Head: 41.24% // Increases project coverage by +0.22% 🎉

Coverage data is based on head (1040225) compared to base (aded93d).
Patch coverage: 78.57% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff              @@
##           RC_v1.5.0     #208      +/-   ##
=============================================
+ Coverage      41.01%   41.24%   +0.22%     
=============================================
  Files             14       14              
  Lines           2321     2335      +14     
=============================================
+ Hits             952      963      +11     
- Misses          1369     1372       +3     

Flag	Coverage Δ
unittests	41.24% <78.57%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tobac/feature_detection.py	80.61% <78.57%> (-0.16%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[snilsn]

Very good work @JuliaKukulies, a great addition. I found one issue with sorting for the dictionary case and have a suggestion for that.

[snilsn]

Thank fro these little changes @JuliaKukulies , I'm approving this.

[freemansw1]

Thanks @JuliaKukulies! I'm happy with this as-is. Feel free to merge when you're ready.

[JuliaKukulies]

Thanks @freemansw1 and @snilsn for reviewing this PR!