WCS fixes

There were issues with pyavm's treatment of `cd` (it was ignored).  I've now done the simplest thing for interpreting `cd` when present:

`cd = [ [ cos(crota), -sin(crota) ] , [ sin(crota), cos(crota)] ] `

so 

`self.Spatial.Rotation = np.arctan2(-wcs.wcs.cd[0,1],wcs.wcs.cd[0,0])`