Creates a new function that unifies a list of GraphFrames into a single GraphFrame

[ilumsden]

This PR implements a new function called unify_ensemble that takes a list of GraphFrame objects with equal graphs and returns a new GraphFrame containing the data of all the inputs. In the output data, a new DataFrame column, called dataset, is added that informs the user which GraphFrame that row came from. If the dataset attribute of the GraphFrame (explained below) is set, that value will be used for the corresponding rows in the output. Otherwise, the string "gframe_#" is used, with "#" being replaced by the index of the GraphFrame in the input list.

To help link output data to input data, this PR also adds a new dataset attribute to the GraphFrame class and a graphframe_reader decorator to help set this attribute. The dataset attribute is meant to be a string that labels the GraphFrame. For most readers, this attribute will be set automatically by the graphframe_reader decorator. This decorator is meant to be applied to from_X static methods in the GraphFrame class. This decorator does 3 things:

1. Runs the from_X function it decorates
2. If the from_X function did not set the dataset attribute and the first argument to from_X is a string, this first argument will be considered a path to the read data, and it will be used to set dataset
3. Returns the (potentially) modified GraphFrame produced by from_X

[ilumsden]

Closing because this functionality is going elsewhere