Packmol-step

* packmol-step-installer now uses the new scheme, which supports both Conda and
Docker installation.
* Added seamm-packmol Docker image

* Added the ability to work in Docker containers.

2023.9.6 -- Using fractional coordinates for periodic systems.
* By convention, SEAMM is using fractional coordinates for periodic systems. The
PACKMOL step was creating periodic structures with Cartesian coordinates, which
can be a bit confusing, though it did not affect any reauls. This change fixes the
issue.

2023.2.15 -- Restructured documentation and moved to new theme.

Added ability to solvate molecules and increased the options to include

* spherical regions
* cubic and rectangular regions
* cubic and rectangular unit cells for periodic systems
* input molecules from any combination of system/configurations and SMILES
* reworked GUI to be more intuitive.

2021.11.27 -- Fixed bug with atom types.

2021.10.25 -- Bugfix for problems with ideal gas approach.

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.