Add support for missing value cleaner

[rastala]

Add an Estimator that computes a missing value replacement value, such as mean, median or mode, for training data. The Estimator then produces a Model that can be applied to replace missing values.

The missing value cleaner should support one or more input columns. Different types should be supported as follows:

• Floating point numbers: mean, median, mode
• Integer numbers: median, mode
• Strings and categoricals: model
• Vectors and other composite types: not supported

The missing value cleaner should be a PipelineStage so it is compatible with SparkML pipelines.

[drdarshan]

Closing since the basic missing value cleaner transform is now merged in #22