NOTE:When there are no registered visualisation clients, this predicate fails with all arguments, and has no effect.
If ViewableName is the name of an existent viewable, SizeList is unified with a list of integers, each integer being the number of elements in one of the viewable's dimensions.
[Assuming that at least one visualisation client is registered]
	Success:
	viewable_create(v1, [], 
			array([flexible, flexible, flexible], any)), 
	viewable_size(v1, X).
        [gives X = [0,0,0]]
	viewable_create(v1, []([](R,S), [](Q,W)), 
			array([fixed, fixed], any)), 
	viewable_size(v1, X).
        [gives X = [2,2]]
	Failure:
	viewable_size(no, X).
	viewable_create(v1, []([](R,S), [](Q,W)), 
			array([fixed, fixed], any)), 
	viewable_size(v1, [1,2]).
        Exceptions raised:
	viewable_size(_, X).
        [gives error 4]
	viewable_size(123, X).
        [gives error 4]
	viewable_create(v1, [X, Y, Z]), 
	viewable_size(v1, q).
        [gives error 5]
	viewable_create(v1, [X, Y, Z]), 
	viewable_size(v1, [a]).
        [gives error 5]