Columns not appearing in VCFs without full headers

[ihodes]

This run should have a GT column, which is found in extant_columns in the DB, but doesn't appear in the table. This may be because there is no header in the VCF, but we still know the columns present, so we should be able to display it.

Note issue https://github.com/hammerlab/cycledash/issues/592 is also a symptom of not handling header issues well: if there's no header, nothing works (this is probably okay; we should just report this to the user instead of crashing).