Removed reading and parsing the special clinsig file

The file is parsed during import, but the information is no longer
being used.
Several improvements in loading VCF data

- a platform in de_gpl_info is used or created
- the platform is referenced from dataset and subject_sample_mapping
- sample_cd in subject_sample_mapping is filled
Bugfix to have the reference column reflect the correct value
Now also importing VCF header metadata
Updated readme according to earlier changes
Merge pull request #11 from thehyve/improvements/FT-711-vcf-improvements

Improvements/ft 711 vcf improvements