convert2geno: work with matrix not vector

 - Was working with IntegerVector and then copying to IntegerMatrix,
   'cause I'd not figured out how to deal with a matrix as if it's a
   vector.

 - IntegerMatrix has no .size(), but you can get
      int n = mat.nrow() * mat.ncol()
   then do like for(int i=0; i<n; i++) mat[i] = ...