Spherical-geometry

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- Fix incorrect query of astropy version information to deal with
deprecation of ``_naxis1`` and ``_naxis2`` WCS attributes. [165]

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- Fixed a bug introduced in version ``1.2.8``.

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- Add backwards compatibility with ``astropy 3.1.0``: ``<3.1.0`` uses
``wcs._naxis`` and ``>=3.1.0`` uses ``wcs.pixel_shape``.

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- Restored ``_naxis``, ``pixel_shape`` not ready yet.

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- Replaced ``_naxis`` with ``pixel_shape``.

- Updated ``README`` file.

- Removed debugging code from ``graph.py``.

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- Added a method to create a polygon from the convex hull of a list
of points.