iFit/dimensions

 Find number of dimensions and extent along each dimension of
 the signal arrays. 
 - If 0D sqw object, nd=0,  sz=zeros(1,0) (nb: []==zeros(0,0))
 - if 1D sqw object, nd=1,  sz=n1
 - If 2D sqw object, nd=2,  sz=[n1,n2]
 - If 3D sqw object, nd=3,  sz=[n1,n2,n3]   even if n3=1
 - If 4D sqw object, nd=4,  sz=[n1,n2,n3,n4]  even if n4=1

 The convention is that size(sz)=1 x ndim

   >> [nd,sz]=dimensions(w)