Support for charge and spin multiplicity

Currently in reading from MOPAC files. Also added support for active spaces via OPEN(n1,n2) and ROOT= keywords.

Added Write Structure Step

Added the ability to write structures, and improved the options for handling of the system & configuration when reading structures.

Streamlined and improved the documentation.

Streamlined the documentation, adding a description for the plug-in.
Also tidied up the description for PyPI, which is used by the new installer, and added keywords to help with searches.

Updated to be compatible with the new system classes.

2021.2.4

Fixed bug left in CI files.

Moved to new system and options handling

And also new CI/CD using GitHub Actions.

Removed leftover debugging print.

Merge pull request #19 from molssi-seamm/bugfix

Removed leftover debug print statement.

Added plotting capabilities.

Consolidating a number of changes, but the noticeable user visible change is the addition of Plotly plots, which are currently used in LAMMPS.

Fixed formatting and bugs with location of MOPAC

0.71

Bug in finding MOPAC exec

Initial, working version.

0.7

First draft of mopac files