Changes:
LabelMapperRange now returns LabelMapperRange._no_label when the key is
not within any range. [#71]
LabelMapperDict now returns LabelMapperDict._no_label when the key does
not match. [#72]
Replaced domain with bounding_box. [#74]
Added a LabelMapper model where mapper is an instance of
~astropy.modeling.core.Model. [#78]
Evaluating a WCS with bounding box was moved to astropy.modeling. [#86]
RegionsSelector now handles the case when a label does not have a corresponding
transform and returns RegionsSelector.undefined_transform_value. [#86]
GWCS now deals with axes types which are neither celestial nor spectral as "unknown"
and creates a transform equivalent to the FITS linear transform. [#92
