Perform a lexsort and return the sort indices and shape as a tuple.

def flds_firstsort(d):
    '''
    Perform a lexsort and return the sort indices and shape as a tuple.
    '''
    shape = [ len( np.unique(d[l]) )
              for l in ['xs', 'ys', 'zs'] ];
    si = np.lexsort((d['z'],d['y'],d['x']));
    return si,shape;