reuse objective results if outputs are exactly the same as a previous run

[blahah]

Currently, objectives are run for the output of every target run. In some use cases (e.g. transcriptome assembly), multiple different parameter sets will result in exactly the same assembly. Let's take md5 hashes of all output files, and use a set of those hashes as the key in a hash of objective scores. Could potentially shave hours off a run for computationally intensive objectives.

[blahah]

md5 = Digest::MD5.file('path_to_file').hexdigest

<3 you Ruby