Spherical-geometry

====================

- The public methods in ``SingleSphericalPolygon`` now match the methods in
``SphericalPolygon`` so that objects of either type can be used
interchangably (for the most part.) ``SphericalPolygon`` now subclasses
``SingleSphericalPolygon``.

====================

- Every method with ``lonlat`` in its name now has an alias with ``lonlat``
replaced by ``radec``.

- The class ``_SingleSphericalPolygon`` has been renamed to
``SingleSphericalPolygon``. The former name has been retained as an alias.

- The from_lonlat (and from_radec) method is now available in
``SingleSphericalPolygon`` as well as ``SphericalPolygon``.

- The methods ``iter_polygons_flat`` have been renamed to to ``__iter__``. The
former name has been retained as an alias.