v2019.6.20

• New interface class (@sivonxay, @kylebystrom, @shyamd)
• Updates to SlabGenerator (@richardtran415)
• Updates to PiezoTensor (@dongsenfo)
• Add support for parsing on-site density matrix to Outcar (@mkhorton, @mhsiron, @clegaspi)
• Fixes for magnetic space groups (@simonward86)
• Fixes for Lobster class (@JaGeo)
• Fix for FEFF (@stevetorr)
• Fix for Waveder (@JRSuckert)