AddScalar and ProdScalar with scalar output nodes

[oir]

Currently scalar operators expect actual scalars (e.g. Reals). However they should allow for size one (or maybe strictly rank-zero) tensor nodes coming from another computation.

E.g:

NodePtr<> x, y;
auto z = Sum(x) * y;

Here Sum(x) is a size one tensor (possibly rank 0) and it is meaningful to use it as a multiplication operand, but there is no such mechanism.