Seemlessly pass three dimensional arrays between between R and Rcpp.  I found a function called threeDIndex on a forum written for Rcpp  but it did not convert from R to  Rcpp smoothly. I needed to be passing these 3D arrays back and forth not just using them in Rcpp.  I re-wrote the threeDIndex code (mythreeDIndex code) that works seemlessly with the R coverterts c() and array().