Improve finding of screening cluster size

[PhilippRue]

So far the default value for the screening cluster size in the KKR code (parameter RCLSUTZ and RCLSUTXY) is set according to our experience with previous systems. This can be improved by automatically investigating the partial norm (see Wildberger PhD, page 48) of the structural Green function. In this way the size of the screening cluster could be determined automatically.