Resolved line distribution issues and additional test coverage for `RecurrencePlot`

[fkuehlein]

• Found and resolved some inconsistencies in 7fb6b01.
• Added tests to test_recurrence_plot.py as of a6f043d, trying not do overdo the parametrization-thing, as is hard to combine with comparison of numerical results without making everything poorly readable. Parametrization of fixtures may be cut even further as edge cases are covered by additional tests, but computational cost is small so I thought why not keep them.
• Finally, I took the liberty of a7ab710 to make the coverage report look nicer, hope that's alright :-)

See #166 for some notes on previous commits.

[codecov[bot]]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (f374ffd) 58.42% compared to head (601af09) 60.72%.

Files	Patch %	Lines
src/pyunicorn/timeseries/recurrence_plot.py	88.33%	7 Missing :warning:
src/pyunicorn/timeseries/cross_recurrence_plot.py	50.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #209 +/- ## ========================================== + Coverage 58.42% 60.72% +2.29% ========================================== Files 44 44 Lines 6369 6357 -12 ========================================== + Hits 3721 3860 +139 + Misses 2648 2497 -151 ```

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[ntfrgl]

@fkuehlein, thank you for implementing and testing the required changes! Could you please carefully review my updates above before we merge? I found some minor remaining bugs, extended your tests to the "sparse" RQA use case, and refactored the Cython implementation to significantly reduce its redundancy. Hopefully, going forward, this will also make it easier to extend RQA methods and other highly parametric functions.

[fkuehlein]

Cool, thank you @ntfrgl! Great to lose some weigth in that Cython module especially, should be helpful for future enhancements indeed. It all looks good to me.