add external dft20 kkr slides

fig/external/presentations/rwth-dft20/README.org

+Lectures from the course "Density Functional Theory and Electronic Strucure",
+given in WS 2020/2021 by Prof. Stefan Blügel at RWTH Aachen University.

fig/external/presentations/rwth-dft20/lecture-20-kkr/README.org

+KKR guest lecture given by [[https://scholar.google.com/citations?user=D7qvTVIAAAAJ][Phivos Mavropoulos]], National and Kapodistrian
+University of Athens, Physics Department, WS 2020 / 2021.