Dataframe name

[jeh362]

closes: #469

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Issue An error is raised whenever a dataframe is created without a dataframe_name parameter. Visualizations should be able to be instantiated without any arguments other than their experiments.

Changes Updated dataframe_plot.py to default dataframe_name to None. If no dataframe_name is passed in, then the dataframe_name is set to the name of the dataframe logged to the the first experiment.

[stephenpardy]

Is this PR dependent on #471 ?

[ryanSoley]

will address later if necessary