Add function for unphasing genotypes

[athos]

This PR adds a utility function for unphasing genotypes in the VCF format.

The function takes and returns a parsed genotype, so if you want to unphase an unparsed (string) genotype, you'll need to use this function together with parse-genotype and stringify-genotype:

(require '[cljam.io.vcf.util :as vcf.util])

(-> (vcf.util/parse-genotype "1|0")
    vcf.util/unphase-genotype
    vcf.util/stringify-genotype)
;=> "0/1"

(-> (vcf.util/parse-genotype "2|1|0")
    vcf.util/unphase-genotype
    vcf.util/stringify-genotype)
;=> "0/1/2"

[codecov]

Codecov Report

Merging #275 (6a5a537) into master (edecb9d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #275   +/-   ##
=======================================
  Coverage   88.85%   88.86%           
=======================================
  Files          78       78           
  Lines        6496     6501    +5     
  Branches      456      456           
=======================================
+ Hits         5772     5777    +5     
  Misses        268      268           
  Partials      456      456           

Impacted Files	Coverage Δ
src/cljam/io/vcf/util.clj	92.77% <100.00%> (+0.14%)	⬆️

[alumi]

Thank you for adding this feature! Looks good to me 👍
I have a minor suggestion for a possible improvement.

[niyarin]

LGTM