Added CheckTree

• Added the CheckTree widget, similar to that in Tix, for handling a tree of checkboxes.
• Added the labelpos option to LabeledWidget to control placement of the label.
• Added 1/[length] units for reciprocal space measurements.

Added widgets for searching

Added the SearchCriteria widget for defining a general search.

Support needed for PyXtal

Added units for area, which is needed for PyXtal.

Internal Release

Updated CI
Switched to black
Added support files to the release so that conda-forge build can work.

Enhancements to PeriodicTable

Added control for active/inactive elements and for coloring of element symbol labels for selected sets of elements in support of DFTB+ plug-in.

Expanded handling of results in two column widget.

Merge pull request #9 from molssi-seamm/psi4

Added clear function to completely clear the scrolled_columns widget.

Added PeriodicTable widget for OpenKIM & BSE support.

Merge pull request #8 from molssi-seamm/periodic_table

Added the PeriodicTable widget

Adding PropertyTable widget

Added the PropertyTable widget to support changes in LAMMPS to automate convergence of properties to a desired accuracy instead of a fixed run length.

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.

End of 2019

This release contains the fixes for the "Cancel" button problem, wherein clicking Cancel did not update the GUI.