Decide whether to use pandas categorical data types or stick with objects

[joelostblom]

The main benefit of the categorical data type over objects is in ordering and size comparisons. It is a best practice but it is unclear how much we will actually leverage any of the benefits in dsci100 or if it is just one more thing to learn. The next version of altair will likely inherent pandas categorical ordering when present, but I don't think anything in scikit learn respects it.