WCS errors for unequal step-sizes

Gris Coordinates recovered from the first wcs sometimes will go in the wrong direction. Also ther is an offset between the coordinates fed into the WCS generator and the ones reconstructed from the header. It's something with the rotation of the different step widths in X and Y direction. Cant't figure it out, spent one week now.

to reproduce run test_coords /test_wcs for commit 50ee44ce