gappa v0.6.0

This is the release that accompanies the publication of our application note

Genesis and Gappa: processing, analyzing and visualizing phylogenetic (placement) data.
Lucas Czech, Pierre Barbera, and Alexandros Stamatakis.
Bioinformatics, 2020. https://doi.org/10.1093/bioinformatics/btaa070

which is now the official reference to cite when using genesis and gappa.