struct IndexOperatorHelper

This mess is needed to expand the parameters_ tuple into individual

arguments to the ElementView constructor. If parameters_ has M elements,

then:

IndexOperatorHelper

<false

>::ConstructElement() calls

IndexOperatorHelper

<false

, 0>::ConstructElement(), which calls

IndexOperatorHelper

<false

, 0, 1>::ConstructElement(), and so on, up to

IndexOperatorHelper

<false

, 0, 1, ..., M-1>::ConstructElement(), which calls

IndexOperatorHelper

<true

, 0, 1, ..., M>::ConstructElement()

That last call will resolve to the second, specialized version of

IndexOperatorHelper. That version's ConstructElement() uses

`std::get

<N

>(parameters)...`, which will be expanded into

`std::get

<

0>(parameters), std::get

<

1>(parameters), std::get

<

2>(parameters),

..., std::get

<M

>(parameters)`.

If there are 0 parameters, then operator[]() will call

IndexOperatorHelper

<true

>::ConstructElement(), which still works --

`std::get

<N

>(parameters)...,` will be replaced by ``.

In C++14, a lot of this can be replaced by std::index_sequence_of, and in

C++17 it can be replaced with std::apply and a lambda.

An alternate solution would be to force each parameterized view to have a

constructor that accepts a tuple, instead of individual parameters, but

that (further) complicates the matrix of constructors for view types.