added volvox example vcf data
factor generic configuration stuff out of BlockBased into a new base class JBrowse/Component that will be used by more than just tracks
mostly-working Tabix index reader and new BGZBlob model for reading bgzipped files
tabix indexes actually do use virtual offsets like bam indexes
mostly-working TabixIndexedFile store, which reads lines out of a tabix-indexed file
bugfix to block merging in TabixIndex
make TabixIndex keep track of the first file offset where data is
add getRefSeqs method to TabixIndex
tweak TabixIndexedFile interface, rename `fetch` to `getLines`, add an `indexLoaded` property the gives access to the index-loading promise
factor stats estimation out of BAM store into GlobalStatsEstimationMixin
added VCFTabix store.  working and emitting features.  still needs deeper parsing.
make HTMLFeatures show the description if at the appropriate scale, even if the feature e has no name (label)
fix vcf header parsing
tweak vcf feature output
bugfix to VCF range filtering
parse more information out of the VCF header
continuing to enrich VCF parsing, add some updates for spec v4.1
parse VCF genotypes
add ALT description to VCF feature description string
update volvox vcf test data to have more metadata for testing