Bad defaults in MoveStageAPI

Currently, an absolute move with any of the coordinates missing will set the remaining coordinates to zero. This behaviour only makes sense for relative moves and could be a major problem for absolute ones. It's not currently an issue as the available clients all make 3 axis moves anyway, but should be fixed. Easiest would just be to remove the missing=0 terms and force the fields always to be specified - we can relax that as and when we can do so robustly.