Return a `processFeatures` method

Add a `processFeatures` method among the return values of `loadCsv`. This method, given a tensor *or* array of values, applies the same processing applied to the feature columns from the CSV file. It should accept a second, optional parameter to return a tensor or a good-old JS array.

Introducing a way to serialize and deserialize this method would be a nice addition. Loading it alongside loading a model from disk, passing it to a client over from the servers, etc., are all very valid use cases.