Dftbplus-step

* Added handling of space group symmetry.
* Automatically use the primitive cell when it is different from the conventional cell.
* Calculate and graph the DOS when running the energy or optimization.
* Added a band structure sub-step to calculate and graph the band structure. This is
an initial, working version, but needs considerable enhancement.
* To accomplish the above, restructured the code significantly and moved the actual
execution of DFTB+ to the appropriate sub-steps. This is need to support e.g. band
structure which requires two sequential calculations, the first to calculate the
charge density and the second to get the band structure from the fixed charge
density.

* Updated to the latest version of DFTB+ (21.2), which made large changes in how
optimizations are handled.
* Updated the structure handling to give the standard options for where to put the
modified configuration and how to name it.
* Added enhancement to calculate the electronic energy of formation, and added the
reference energies to the metadata for the main 3ob and mio datasets.

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

2021.10.13 -- Minor fixes and format issues.
Also updated for Python 3.8 and 3.9

2021.6.5 -- Added installation of Slater-Koster files.

2021.6.4 -- Updated for new command-line argument handling.
Corrected the default path for the Slater-Koster functions.

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

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* Updated the README file to give a better description.
* Updated the short description in setup.py to work with the new installer.
* Added keywords for better searchability.

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* Internal Release

- Compatible with the enhance version of MolSystem classes.