Execution

Just for feedback
It's actually intended to be the common implementation behind the `MaestroExecution[YourType]` trait and the `MaestroExecution` object. The MaestroExecution object is intended to be where you do your wildcard imports from, or import as {MaestroExecution => Ex}, whatever you prefer.

I noticed you imported from MaestroExecutionBase in etl.psgl though. I can't remember why you needed to do that?