#38 keep custom rginfo lines in vcf headers in synchrony with sample names

[Wolfgang Maier]

varcall is currently the only tool writing the MiModD-specific rginfo records to vcf headers. Other tools (i.e., vcf-filter and varextract), however, may change the sample names declared in a vcf file without updating the rginfo records.
The resulting inconsistency is silently ignored by the pyvcf module, but may cause trouble at some point.