Support phase and image centre being different

[samaloney]

Take a visibility with a phase and map centre of 0,0 and create and image between -5 to +5 in x and y the same image should be created with a phase centre of -5, -5, map centre of 0,0 created between 0 +11 and 0, +11 (if the visibilities are adjusted correctly)

It's the product of xu and yv that matters for the image/array values the centre 'coordinate' is need for coordinate aware transforms, plots etc