Plug-in for SEAMM for building fluid systems with PACKMOL.

Provides a step in a flowchart for packing reasonably small molecules into a periodic cell using PACKMOL. This is used for preparing models of liquids and gases with one or more components given the stoichiometry. The size if the resulting model is specified by giving two independent parameters such as the density and number of molecule, or volume and total number of atoms. In addition, using the ideal gas model you can specify the temperature, pressure and either the volume, length of the cell side, or total number of molecules or atoms to create the modelFunding including NSF OAC-1547580 and CHE-2136142 grant