Add Protocol.validate method that takes same inputs as create; add a passthrough in Transformation

[dotsdl]

In practice, a Protocol can fail during execution due to poor input choices to their create methods, such as incompatibilities between stateA and stateB, mapping issues, or conflicts with any of these and settings choices. This can trip users up, cost them time in diagnosing problems in (potentially remote) execution, and waste expensive compute on transformations that are certain to fail.

One option is to encourage Protocol authors to add validation checks for known limitations like the above to their _create methods, but this would slow down execution by remote execution systems every time the create method is called.

Instead, we propose adding a distinct validate method (with corresponding _validate method that Protocol authors specify) that features largely the same signature as create. This method would allow Protocol authors to specify (reasonably) rapid validation code specific to their Protocol that can catch obvious issues before performing potentially expensive compute on a set of inputs that are bound to fail.

[jthorton]

I give +1 for this idea. We want a rapid validation method that checks common failure points while creating the transformation to make it fail during network construction rather than at DAG creation.