Windpowerlib

This release mainly updates the code basis to work for newer versions of python that only work with newer versions of pandas. To that end the support for python 3.6 to python 3.9 is discontinued. However, python 3.9 should still work, it is just not tested anymore.

Further, github actions to run tests automatically when changes are pushed to github are added.

Some improvements have been done that make the windpowerlib faster.
Furthermore, it is easier to use one's own data with the windpowerlib.

See the changelog for more information.

We are happy to present to you the newly revised windpowerlib! Major changes are the API revision of the WindTurbine and WindFarm classes that make the usage of the windpowerlib more convenient.
Have a look at the [what's new](https://windpowerlib.readthedocs.io/en/stable/whats_new.html) for more information on what has changed.

We are also happy to see that the number of users of the windpowerlib is steadily increasing. We therefore find it important to keep the API changes to a minimum from now on and announce changes well in advance. We also decided to support v0.1.3 for some more time in case you don't want to upgrade your code to v0.2.0.

The oedb change the name of the turbine table, so we had to adapt it in the windpowerlib. Update your windpowerlib using:

`pip install windpowerlib --upgrade`

We will soon release v0.2.0. Stay tuned :smile:

From the “First single zero release” power (coefficient) curves are loaded from the OpenEnergy Database (oedb) instead of from csv files. This release permits the users to work offline again as the turbine data provided by the oedb is available as csv files in the repository.
Moreover, from this release on we are working with deprecation warnings to draw our user's attention to important changes.

Windpowerlib v0.1.0 did not have an upper limit for the pandas package. Due to API changes in pandas v.0.24.0 the examples did not work anymore. We fixed the code to work with the new pandas version and set upper limits for required packages.