Add basic Cusum

[jchester]

• [x] Cumulative sum (Cₙ)
• [x] Upper cumulative sum (C⁺)
• [x] Lower cumulative sum (C⁻)
• [x] Allow different values of allowance (K) and decision interval (H) for C⁺ and C⁻
• [x] Rename individual_measurement_statistics_ewma as it is now used in Cusum calculations as well.
• [x] Add documentary comments
• [ ] Deal with multi-measurement samples (right now it is written for individual measurements).
  • [ ] Update documentary comments
• [ ] Update readme

I won't do fast initial response in this one, but may reserve it for later. No intention of ever trying figure out how to do a V-mask procedure.

[jchester]

N⁺/N⁻ have almost the same problem as C⁺/C⁻ had, which is that the ideal solution would be a field that is defined in terms of itself. They also share that if PostgreSQL had a reset when on window functions, as Teradata does, the problem would be much easier to solve.

For now I am going to break out N⁺/N⁻ into their own issue so that I can move on to configurable decision intervals (H).

[jchester]

I think it would be best to hold off on this until #22 is done, because the way that sample averages are joined will change substantially.