`DataframePlot` - don't require `dataframe_name`

[ryanSoley]

Is your enhancement request related to a problem? Please describe

it should be possible to instantiate visualizations without any arguments other than their experiments

Describe the solution you'd like

dataframe_name should default to None

https://github.com/capitalone/rubicon-ml/blob/main/rubicon_ml/viz/dataframe_plot.py#L46

if no dataframe_name is passed in, assume the dataframe requested is the first dataframe in the first experiment, e.g. self.dataframe_name = self.experiments[0].dataframes()[0].name. if experiments[0] has no dataframes, raise an error

[ryanSoley]

depends on #468