Find features that qualify as training data

[hafezissa]

Expanding on ISSUE 19, we want to narrow down the features for training. Ideally, there should be an eligibility criteria for whether a feature can be used for training. We have an idea of what that criteria should look like:

• Look only at float64 and int64 -- some float64 should be converted to integers
• Disregard features that have strictly one unique value, ie. all ones, all zeros, all nulls
• Calculate the percentage of NULL for each feature, if the percentage is higher than 1/3 (this threshold is just an example) then disregard that feature
• Any other ways we can eliminate these features?

DoD: Number/List of features that pass the criteria.