Packmol-step

This release adds

* the ability to directly generate multiple different molecules from SMILES, giving
the stoichiometry for packing into the fluid box.
* specifying the desired cell with the temperature and pressure, along with one of
the volume, length of the box side, number of molecules or number of atoms. This
uses the ideal gas law, so will only be reasonable at higher temperatures and lower
densities.

Additionally, internally the release adds integration tests to check whether the module works properly in the SEAMM environment.

2021.5.25 -- Added installer for Packmol executable.

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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.