Mopac-step

Calculates and writes the Hessian (force constant) matrix to disk. Works for both
molecular and periodic systems, and provides an option to control which parts of the
Hessian matrix are written. Defaults to the full matrix. Also provides options to
control the units of the output, with default of N/m for the atom block of the
Hessian as well as the atom-cell off-diagonal block, and GPa for the cell block.

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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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* Updated for compatibility with the new system classes in MolSystem
2021.2.2 release.

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* Internal: switching CI from TravisCI to GitHub Actions, and in the
process moving documentation from ReadTheDocs to GitHub Pages where
it is consolidated with the main SEAMM documentation.

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* Updated to be compatible with the new command-line argument
handling.

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* Updated to handle citations using the new framework.