Tool output caching

[mfitzp]

Some long-running calculations would be nice to 'skip' using a local cache of processed data. The cache should hash the inputs (data) and config (settings) for a given tool, which taken together should predict the output. The runner can check for the existence of a cached result before initiating the run and return that data instead.

Implement cache as a store of pickled objects.

Tools can opt-out of caching.