Featorig

The feature index class provides two functions: ``gt_feature_index_get_range_for_seqid``, which returns the range inferred from features; and ``gt_feature_index_get_orig_range_for_seqid``, which returns the range specified by the original input (such as specified by the ``##sequence-region`` pragmas in a GFF3 file).

This commit extends the ``GtFeatureInStream`` API so that the programmer can specify which function to use when delivering region nodes into a stream from a feature index.