Sdss-coordio

----------------

* When ``Calibration`` does not have any files, the data frames are set to empty instead of ``None``.

----------------

* Use measured alpha and beta offsets when transforming from tangent to positioner.
* Replace error in ``iauPmsafe`` with warning.
* Add ``RoughTransform`` and ``ZhaoBurgeTransform`` (11).
* Undo changes to ``wokToTangentArr``. Reverted to only supporting one holeID per array (11).
* Add a ``Calibration`` class to store all active calibrations, allowing for concatenation of different site calibrations (12).

-------------------
C++ implementation of wok, tangent, positioner transforms. Improvements to packaging.

--------------------

First tagged version