Clarify and harmonise mean of centre of offset

[samaloney]

xyoffset is the coordinate offset of the phase centre (0,0)

should be able to get the same result with and xyoffset of phase centre (0,0) and the correctly choosen u,v ranges and xyoffset of (0,0)

[samaloney]

So there is really two different concepts here (see #57) but in the majority of case the centre of the map and the phase centre will be the same so you can specify the map size as a symmetric value e.g -5 to 5