Implement active methods more simply

[richfitz]

Instead of doing the if/else in R and having two C++ functions, do the if/else in C++ and pass along either something to indicate missing (3rd arg as logical result of missing(value)) or detect missingness in C++ http://stackoverflow.com/questions/29283954/check-if-there-are-missing-arguments-in-enclosing-call-using-c-rcpp