Add support for writing a selection of SNP call data to a plink binary format file. Useful to export data for analysis with tools like ADMIXTURE. Prototype implementation [here](https://colab.research.google.com/drive/1E95Dn-epsYfm3Krg6Okl7gpPfAN_crf7?usp=sharing).
