Get all sklearn estimator checks passing

[aazuspan]

As discussed in #20, we've run into a number of issues that probably would have been caught earlier with the built-in test suite in the sklearn.utils.estimator_checks module, so it's probably in our best interest to focus on getting those passing. This should also potentially allow us to remove some redundant tests from test_estimators.py.

Note that it seems like some valuable tests like check_dataframe_column_names_consistency aren't running automatically, so we'll need to make sure we run everything that's relevant.

EDIT: Our tests currently contain commented-out checks for our estimators, but it looks like there are also transformer checks that we should run. In fact, maybe it would make sense to get our transformers passing first since those will affect the estimator results?

Another aspect that I'm not very familiar with but that we'll probably need to dive into for this is the sklearn tag system, which allows you to specify different compatibility options for estimators and transformers that inform which checks will be run.

[aazuspan]

Resolved by #44