MAINT: Refactor caching (#124, #148, #153)

[ntfrgl]

This PR fully implements:

• the suggestions (1-3) in this comment, i.e., a new mix-in class Cached with a decorator @Cached.method(),
• a bare minimum but robust version of suggestion (4), i.e., a detailed behavioral test of the mix-in itself which leaves subclass-specific tests to future work,
• a resolution of the incorrect caching behavior in #153, and
• a resolution of another minor issue due to obsolete caching attributes.

Obviously, there is still more refactoring to be done to make the caching system consistent across the project, but this PR should deliver on the main concern of #148, namely a well-behaved caching decorator. The last commit message should provide enough context for reviewing. @fkuehlein, please raise any concerns you might have, since you are expected to maintain this code going forward.

[codecov[bot]]

Codecov Report

Attention: 54 lines in your changes are missing coverage. Please review.

Comparison is base (2739894) 62.82% compared to head (415e2bf) 63.76%.

Files	Patch %	Lines
src/pyunicorn/climate/havlin.py	52.00%	12 Missing :warning:
src/pyunicorn/timeseries/recurrence_plot.py	85.89%	11 Missing :warning:
src/pyunicorn/core/cache.py	85.71%	8 Missing :warning:
src/pyunicorn/timeseries/joint_recurrence_plot.py	50.00%	5 Missing :warning:
src/pyunicorn/climate/climate_network.py	80.00%	4 Missing :warning:
src/pyunicorn/climate/hilbert.py	73.33%	4 Missing :warning:
src/pyunicorn/core/network.py	96.77%	4 Missing :warning:
src/pyunicorn/climate/mutual_info.py	76.92%	3 Missing :warning:
src/pyunicorn/climate/climate_data.py	95.83%	1 Missing :warning:
src/pyunicorn/core/geo_network.py	90.90%	1 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #219 +/- ## ========================================== + Coverage 62.82% 63.76% +0.93% ========================================== Files 43 44 +1 Lines 6220 6221 +1 ========================================== + Hits 3908 3967 +59 + Misses 2312 2254 -58 ```

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[fkuehlein]

Thanks a lot @ntfrgl for all the work put into implementing this and also for all the little cleanups and refactors along the way!

I took a bit of time to understand the structure and functionality of the new Cached class in detail, although its application appears straightforward indeed. I think I'll be able to maintain it, continuously include it in further classes and eventually add subclass-specific tests.

I added the remaining minor fixes in the commits above to make sure all checks pass, so this PR should be good to go.

[ntfrgl]

Thank you for the careful review. I'm glad to know the project in your hands.

As a last addition, I slightly simplified the implementation of nested Cached objects in afc6abf, effectively removing some debugging artifacts. But that shouldn't concern the current release.