CV capacity summary statistic

[d-cogswell]

This adds a CV capacity summary statistic for both regular and diagnostic cycles, which is distinct from the CV time and current stats I added previously. I included a regression test.

After merging I would appreciate a re-run of structuring on AWS for PreDiag and PredictionDiagnostics data sets.

[shijingsun-TRI]

LGTM, new functions will show up in the pipeline following the next release of the package.

[ardunn]

tests failed due to a coveralls error, I am rerunning all of them. once they pass we should merge

[ardunn]

I would amend the get_CV_capacity function to throw the correct exceptions when bad input is given; returning None using isinstance checking might cause silent errors down the line