Add caching to pipelines

[allemangD]

Create a decorator that checks os.stat to avoid duplicate work in pipelines.

[ebrahimebrahim]

(Not ready to do this until we have at least two or three components created from #3 )

[ebrahimebrahim]

We are now, after discussing things for #44, more liking the idea of each step handling its own caching with somehting like an overwrite flag.