add functional performance functions snakemake

[galengorski]

This is a cleaned up version of the functional performance calculations with a snakemake wrapper to run the functions. The three functions are:

• create_cleaned_io_data -- generate cleaned up data for functional performance calcs
• calc_it_metrics_for_pairs -- calculate TE, MI, correlation for pairs of sources and sinks during time periods specified
• calc_functional_performance -- using the TE calculated from calc_it_metrics_for_pairs calculate difference between observed and modeled and save to file

[galengorski]

Thanks for the review @jds485 ! I added a function to generalize the number of replicates and I cleaned up some comments as you suggested. If you're ok with it, I'll go ahead and merge it.