Periodic calculations

Added handling of the reciprocal space k-mesh for periodic calculations.

Minor fixes and format issues.

Also updated to Python 3.8 and 3.9.

Added installation of Slater-Koster files.

Merge pull request #4 from molssi-seamm/installer

Finalized installer handling the Slater-Koster files.

Updated for new command-line argument handling.

Updated for the seamm argument parser moving to seamm_util.

Corrected the default path for the Slater-Koster functions.

Added installer for DFTB+ itself

Added a plug-in specific installer that installs DFTB+ in the seamm-dftbplus environment if needed, and sets up the configuration file entries needed.

Improved the description displayed in the new installer.

Updated the short description and keywords to better represent the pl…

…ug-in.

Upgraded to be compatible with the new system description.

Merge pull request #2 from molssi-seamm/system2

Updated for the new system

Initial working version

2020.12.2

Deploying to PyPi regardless of tags.