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.