add `gt_feature_index_get_orig_range_for_seqid()`

This adds `gt_feature_index_get_range_for_seqid()` as a new `GtFeatureIndex` function, delivering the range of the original sequence region node inserted into the index, instead of the region populated by features (cf. `gt_feature_index_get_range_for_seqid()`). This addresses issue #13.