DATAFU-50 SimpleEvalFunc should extend ContextualEvalFunc, have good lifecycle hooks

* SimpleEvalFunc now extends ContextualEvalFunc (rather than plain-old-EvalFunc), letting front end talk to back end
* ContextualEvalFunc has a hook method onReady(in_schema, out_schema) -- this is sugar for UDFs that simply want an opportunity to capture or set values once for a job, rather than legitimately wanting to override outputSchema().
* Updates AliasableEvalFunc to use onReady

https://issues.apache.org/jira/browse/DATAFU-50

Signed-off-by: Matt Hayes <mhayes@linkedin.com>
Correct doc string in ContextualEvalFunc