VSIPL16 — tuple should provide value accessors

VSIPL++ implementations are not required to provide any member
definitions for tuple<>. However, both the old reference implementation
and the Mentor Graphics implementation define static members with the
values of the dimension ordering. These are certainly useful within the
implementations, and are likely to be useful to applications as well.

Recommendation: Require definition of 'dim0', 'dim1', and 'dim2' tuple
members, which match the tuple's template arguments.