support for Plink file format(s) for writing

[bhaller]

It would be good to support Plink file formats for writing, for downstream analysis. Seems less important to support them for reading (and perhaps difficult to wedge the data they contain into SLiM, not sure). This was requested by Diego Ortega Del Vecchyo; maybe I can follow up with him if I end up having questions.

[bhaller]

@dortegadelv Did I find the right username on GitHub for you?

[dortegadelv]

Yes! The username is correct, Ben!

The plink format is used by various programs to do statistical genetics analyses. Since @bhaller is expanding SLiM to do more analyses on complex traits, I thought that users would be interested on using SLiM output to run Plink or other popular software that use Plink files to do complex traits analyses (such as LDAK or GCTA).

I don't think we should use the plink format to read files with SLiM.