convert2geno: force output to be matrix of _integers_
Add tests of convert2geno
convert2geno: change coding for multi-allele case

  - instead of 2^g1 + 2^g2, use 2^(g1-1) + 2^(g2-1)
Move convert2geno from R to c++
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] = ...
Move get_geno from R to c++