Consolidate VCF writing code

Currently there are at least 4 ways we write VCFs

* ```get_vcf_header_lines``` and ```variants_to_vcf._write_sorted_values_to_vcf_file```
* ```write_vcf_from_tuples```
* ```ClassificationExportFormatterVCF```
* ```analysis.views.views_grid._grid_export_vcf``` (uses ```vcf.Writer```)
 
