Make it possible to use operators for serving or batch

[karlhigley]

• [ ] Add the capability to select output formats at the operator and/or ensemble level (np/cp/pandas/cudf etc)
• [ ] Adjust the operators as needed to work with pandas/cudf output
• [ ] Figure out how to handle operators that fetch data from external systems (provide an option to replace that with a cudf join?)
• [ ] ...