Generalise the visibilities to support both a phase center and phase offset

[samaloney]

See the discussion here #67 (comment)

If you want to reconstruct an image centered in [x,y] you can equivalently:

1. have phase_center = [x,y] and create a meshgrid X,Y centerd in [0,0]
2. have phase_center = [0,0] and create a meshgrid X,Y centered in [x,y]
3. In general, have phase_center = [x',y'] and create a meshgrid X,Y centerd in [x - x',y - > y'] (or, maybe, [x' - x,y' - y]; it does not matter for this discussion)

Also given a Visibility $V$ and a new visibility $V' = \phi V$ created by applying and complex phase shift $\phi$ its not possible to go back to $V$ as given only $V'$ as $\phi$ is not stored in $V'`$

So we need to store two items the values used to generate the mesh grid the phase shift as this make 3 and solve the reversibly problem, both can be represented as pairs of $[x, y]$ coordinates.

Possible naming conventions:

phase_reference, phase_offset
phase_center, phase_reference
phase_center, phase_offset

1. phase_center=[x, y], phase_reference=[0,0]
2. phase_center=[0, 0], phase_reference=[x,y]
3. phase_center=[x', y'], phase_reference=[x-x',y-y'] ?