Bar plot

Created by: John-P

A branch off of my abstract-constraints branch which adds a new plot which can plot vertical bars.

Series may also be plotted as stacked bars by setting the stacked kwarg to true. Spacing between groups may be set by setting the spacing kwarg to a float from 0.0 to 1.0.

Data values are plotted against their position index in the list (rather than pandas dataframe index column) and series are plotted together in groups around each index.

Also added a fix for plotting data frames which is currently broken in trappy origin.