Following the pvlib

Due to some changes in the pvlib API we had to adept the feedinlib, too. So use pip3 install feedinlib --upgrade to update the feedinlib and the pvlib.

When we started the feedinlib, the pvlib was a only a toolbox. But things change and now a ModelChain class exist that allows us to shorten the feedinlib code. In cooperation with the pvlib it is planned to move most of feedinlib's extra code directly to the pvlib. This could avoid incompatibility problems in the future.