Add deprecation warnings to Fleur Calculator module

... and corresponding documentation page

This module can now be completely replaced by the calculator implemented in https://github.com/JuDFTteam/ase-fleur

This package is not yet released, since it depends on unreleased features in the ase master branch. But since I will publish the PyPi release as soon as the next ase release is done, it would be nice if that release already refers users to the fleur extension