reconcile IVOA DALI xtype usage with Plane metadata

[pdowler]

this is mainly about Shape and Multipolygon which are in WD-DALI-1.2 but there is still some disagreement between DALI authors.

also, in radio a simple observation is often characterised by a circle (maybe ellipse), so it is plausible that a DerivedObservation that combines data for larger sky coverage would have something like multi-circle or multi-shape.... TBD

[pdowler]

deciding to future proof the model with MultiShape as 1..* Shape(s)