+ make plink.mk more robust
+ make plink.mk more robust

  - ensure that the Makefile "successfully" works when the input vcf
    contains no variants
     - check for an empty input vcf
     - handle the empty input vcf case accordingly (ensure a "empty-vcf"
       file is created
     - make the reformat functionality a function instead of a PHONY
       target
     - use the bash shell
   - ensure that the output directory gets created no matter what

indraniel/yaps2,indraniel/yaps2,indraniel/yaps2