`execute` function

[flahn]

While thinking about the workflows in the 'openeo' package for synchronous and asynchronous calls and also the sample data retrieval for local prototyping, I realized that all functions do mostly the same, conceptionally speaking. My thoughts are now to put all the different workflows into one function that is user controlled. I will leave the other functions as well, but it might be convenient to have a single entrypoint for the "processing" of data.