Add unit tests for clause mark specification and new plotters in implicit_plotter.py and cg_plotter.py

[leijie-wang]

• Clause mark type specification

In Clause.py I added a mark_type parameter to make sure that column ends up as a vis with a certain mark type but haven't written tests to make sure this is actually enforced. Mostly means adding tests to the compiler to make sure the clause results in visualizations that are of this mark type

• new plotters in implicit_plotter.py and cg_plotter.py.
  These might be a little tough to write tests for since they can have a huge number of inputs but it would be good to have some unit tests that make sure that they aren't broken. For example, making sure that when df.describe() is called it produces a box plot and the right history.