New MLManager API

BREAKING CHANGES: MLManager has been completely redesigned and the MLManager class no longer exists.

The new implementation gives users full functionality to the vanilla mlflow api with mlflow.FUNCTION calls, as well as Splice's custom functions through the same module.

Accessing the mlflow module is simply from splicemachine.mlflow_support import *
And to handle artifacts, simply call mlflow.register_splice_context(splice)

MLFlow upgrade:
@abaveja313 and @Ben-Epstein