VSIPL16 — scatter, gather should support multidimensional index views

The gather() and scatter() functions, as defined, are capable of
gathering from or scattering to multidimensional arrays, by virtue of
supporting multidimensional Index variables.

However, they cannot gather to or scatter from multidimensional
arrays, because they explicitly specify only Vectors of Index variables.
The extension to Matrices and Tensors of Index variables, with
corresponding-type gather() output and scatter() input, is obvious and
should be included.