Segmentator v1.6.0

Major changes: GUI elements now render properly also in Matplotlib version 3.x. License is changed to BSD-3-Clause. Normalized graph cut feature is deprecated because of dependency problems. Older versions of Segmentator can still be used in Python 2 and following the instructions on the related wiki page. This feature might be re-implemented from scratch in the future if I have the resources. Minor changes: Fixed nifti header being not copied properly from the source image when using segmentator_filters. Tested and updated other dependencies (numpy, scipy) to more recent versions in setup.py and requirements.txt