text: Title: Cleanup Scalar Converter Code
  Body: We have the original draft of the scaler code uploaded in [this branch](https://github.com/microsoft/hummingbird/tree/cleanup/scaler). All credit here goes to @scnakandala, the original author and brains behind this.
  
  It contains an un-edited implementation of RobustScaler, MaxAbsScaler, MinMaxScaler, and StandardScaler [here](https://github.com/microsoft/hummingbird/blob/cleanup/scaler/hummingbird/ml/operator_converters/scaler.py).  We need to integrate this old code into the current code.
  
  There is a single test file [here](https://github.com/microsoft/hummingbird/blob/cleanup/scaler/tests/test_sklearn_scaler_converter.py) that needs to be cleaned up, expanded, and passing.