add step_average

[dsweber2]

After some discussion, we concluded that it would probably be worthwhile to implement something to enable simple 7 day averaging

[dsweber2]

should probably wait to see what happens in exploration-tooling with averaging

[dsweber2]

So apparently step_bs exists, along with all sorts of other spline methods; pairing with a complete, this would allow us to do way more sophisticated kinds of smoothing easily.

[dajmcdon]

See also the step_epi_growth_rate(). Though there's a bug there (it doesn't operate on test data correctly). #345