Validation step

[empEvil]

The idea here is to add the option for a stage to test if the input it is supposed to run on actually have the data-structure the stage expects.
The idea is that to any stage you can add a method validate that will be run before running the main run method of the stage. The idea is that the user can then write their own validation for each stage. If no validate step is done, it should just pass it by, and continue to the execute part.
If an error is found in the validation it should raise an error and prevent an attempt to run the full stage and thereby waste lots of time.
An example for use case would be in TXPipe where the stage TXTwoPoint requires the use of a NofZ file, this file is only called at the end, and need to be structured in a specific way. The structure is something that has changed, and hence older files might not work, But you won't know until after several hours.

[joezuntz]

Minor changes here only