Virocon

Changes
* Added Windmeier's EW model and all the required changes in the core code of virocon
* Added a model inspired by Windmeier's EW model where the dependence function is slightly changed
* Added the benchmark datasets B and C
* Make use of `random_state` to define a seed for random numbers at all distributions
* Added an example script to compute contours based on all predefined sea state models, `hstz_contour_all_models.py `
* Added tests, pushing our overall test coverage to 84%
* Officially support Python 3.11 and update some packages of `requirements.txt`

Changes
* Fixed a bug where sometimes one design calculation less than what was specified was returned
* Added unit tests, e.g. for design condition calculation

Changes:
* Users can now use any `scipy `distribution in a virocon joint distribution model, https://virocon.readthedocs.io/en/latest/joint_models_estimating_parameters.html#custom-joint-models
* New plotting function is introduced, `plot_histograms_of_interval_distributions`

What's Changed
* Fix docstring problems (links and closing quotes). by ahaselsteiner in https://github.com/virocon-organization/virocon/pull/153
* Downgrade networkx as readthedocs servers (?) does not have any versi… by ahaselsteiner in https://github.com/virocon-organization/virocon/pull/154
* Add config file for readthedocs. by ahaselsteiner in https://github.com/virocon-organization/virocon/pull/155
* Implement Von Mises Distribution by frp09 in https://github.com/virocon-organization/virocon/pull/157


**Full Changelog**: https://github.com/virocon-organization/virocon/compare/2.0.14...2.1.0

* Use GitHub Actions instead of Travis CI
* Minor corrections to the docs (which are now hosted at readthedocs.io)

Now uses the newer numpy version 1.17.4 . This caused some compatibility problems, which we fixed.