Get the group id of the feature. The group id is defined in the GFF1
format; features that share a common group id are presumed to be part
of some logical group. For example, a gene might be represented by several
exon features that all share the same group id.
The exact meaning of a feature's group id, or even its existence, is not guaranteed
by this interface. An understanding of a particular file's data format is necessary to properly
interpret the group id.