input parser

[stephens999]

it is intended that we also allow "input parser" as well as output parser. This is a to-do item... just documenting it here.
Something as an aside/motivation, it might help with @mengyin 's use where she is actually
comparing combinations of methods with different pre-processing steps. She is
currently doing all the different pre-processing steps in the datamaker, which is rather messy.
If we had input parser functionality then this could deal better with this - each
pre-processing step could be implemented as an input parser.

This raises the question of how to deal more generally with multi-step pipelines... the idea of a parser whcih takes one type of file and changes it to another is basically rather general...