support for plink 1.9

[d-laub]

• PLINK 1.9 .bed files -- pgenlib supports them transparently
• .bim files: pending testing
• .fam files: needs prototyping and testing

Possible work: path handling e.g. genoray currently expects a .pgen suffix and then checks for PLINK 2 files. However, all PLINK 1.9 files are valid for PLINK 2.0 and handled transparently, so it is convenient to recapitulate this in genoray.