add ability to generate basic statistics for the individual datasets

[ymahlich]

Create a suite of scripts (maybe as part of the coderdata.utils) that can generate basic statistics on the datasets, specifically in regards to metrics that are vital to balancing train/test/validate splits.

Should output basic info as tables and plots.

[ymahlich]

Two new functions have been implemented in a new module called utils.stats:

• summarize_respones_metric
• plot_response_metric

Both are helper scripts to perform operations on the experiments portion of a CoderData object.
For example uses a new Jupyter notebook has been added to /notebooks.