Improve error handling when 3d astra projector called with no GPU available

[bwohlberg]

While the 3d astra projector linop.xray.astra.XRayTransform3D requires a GPU, it does not include any tests to confirm that this requirement is satisfied; calling a forward or back projection without a GPU results in the rather opaque "Unknown algorithm type" error from astra. Some error checking should be added: at least in the example scripts that make use of this class, and possibly also in the class itself.