Sugar functions: cumprod, cummin, and cummax

[nathan-russell]

I'm preparing a PR that includes sugar counterparts for cumprod, cummin, and cummax in base R, along with the respective unit tests. I don't believe this will affect any existing code, and my unit tests & Travis build ran without hitch, but I wanted to open this issue for the sake of questions, concerns, or general discussion.

Briefly,

• Rcpp::cumprod is templated for INTSXP, REALSXP, and CPLXSXP; and Rcpp::cummin & Rcpp::cummax are templated for INTSXP and REALSXP; in accordance with base::cumprod, base::cummin, and base::cummax, respectively
• All three appear to handle NAs and matrix arguments correctly (re: in the same manner as base R)

[eddelbuettel]

Absolute awesomeness for

• opening issue before code submission
• testing before even that and of course
• new code

Negative points for zero time between issue ticket and pull request negating point 1. You still lead by 2:0 at the half ;-)

[nathan-russell]

On the bright side, I wasn't docked any points for submitting on Read Only Friday : ). But noted - I will space things out more appropriately in the future, and thank you for the quick response,

[eddelbuettel]

This has been merged.