Add Character "Select" Dropdown Feature in Vis.py

[emontp]

Picking up from the new "Character_" columns from Joel's branch, work needs to be done in Vis.py to read in the CSV and do the following:

• Find a way to add to the global _Fields variable/create a new variable list;

• Run the data through same/similar functions to parse the data into similar chunks as existing elements in "_Fields";

• Calculate the geometric mean of characters' speech participation in each chunk;

• Create two button groups using the Bokeh "Select" Widget which will give the user the ability to select two characters at a time;

• Ensure that these new features are incorporated into the ratio feature of the existing code;

• Create a function that builds a stacked area plot to display up to two selected characters, based on the calculations for geometric mean.