Return null if vcf quality string is not number

Null value: Cast to Double instead of double
Refactor existing vcf data row tests
Add more tests for vcf data row

- test when QD=. in info field
- test when info field is null
- test when all quality fields are null
Merge pull request #53 from thehyve/issues/vcf-safe-parse-number

Return null if vcf quality string is not number