Use Pooch to handle testing data retrieval

[Zeitsperre]

In #223, the testing data handling has been completely changed in order to facilitate less redundant and more secure downloading of our testing data. Many of the functions being used are based on those that were once in xarray and seaborn, but those libraries have since migrated to pooch.

Pooch would greatly simplify the way that we handle datasets retrieval, and would likely be much easier to maintain. This tool is worth exploring.