Lagged / Leading percent changes

[dajmcdon]

Create a recipe step that does lagged percent changes.

• Simplest case: given a target time and a reference time, compute (target - reference) / reference. For example, target=0 and reference=7 and signal is cases, we get a column pct_chg_7_cases = (cases_0 - cases_7) / cases_7
• Allow target and reference to be scalars or vectors.
• Assert that Reference > target.
• Allow for padding or some behaviour to deal with 0-valued signal in the reference.
• Allow for log version as well log(cases_0 / cases_7).

This is helpful for #5 .