toil-vg construct fails at building the GBWT index if you haven't specially specified --vcf_phasing files

[ruolin]

require(len(options.vcf_phasing) > 0, 'generating a GBWT requires a VCF with phasing information'). After delete _phasing it works again.

[adamnovak]

I don't think deleting _phasing from the code is the right fix here.

If you want a GBWT, right now you have to feed your VCFs to toil-vg construct via --vcf_phasing (for making the GBWT) in addition to via --vcf (for making the graph). If you turn off the check against --vcf_phasing being empty, you might just get an empty GBWT.

What we ought to do is use --vcf files if --vcf_phasing is not specified.