Placeholder: MLeap Imputer should handle numeric types

[seme0021]

Once SPARK-20604 makes into a release, MLeap should also support numeric types, not just doubles.

Be sure to check if the Spark implementation supports Ints or just [DoubleType, FloatType]

[hollinwilkins]

Automatic casting means this transformer supports all data types that can be cast to a double.