Replace SplitInTwo with LeftSplitInTwo/RightSplitInTwo for precise splitting

Replace SplitInTwo calls with LeftSplitInTwo or RightSplitInTwo depending on the intended split direction. In fastseq_json_header.go, extract rank from suffix without splitting; in biosequenceslice.go and taxid.go, use LeftSplitInTwo to split from the left; add RightSplitInTwo utility function for splitting from the right.

pkg/obiseq/biosequenceslice.go

@@ -195,7 +195,7 @@ func (s *BioSequenceSlice) ExtractTaxonomy(taxonomy *obitax.Taxonomy, seqAsTaxa
 				return nil, fmt.Errorf("sequence %v has no path", s.Id())
 			}
 			last := path[len(path)-1]
-			taxname, _ := obiutils.SplitInTwo(last, ':')
+			taxname, _ := obiutils.LeftSplitInTwo(last, ':')
 			if idx, ok := s.GetIntAttribute("seq_number"); !ok {
 				return nil, errors.New("sequences are not numbered")
 			} else {