Implement reduce algorithm #4

We should implement a reduce algorithm that will reduce a grb::vector to a single value or reduce a grb::matrix to a single value, or vector by row or column.

We had a long discussion in this week's meeting about the API for reduce, and whether to

1. Default an initial value to T{}, as in std::reduce
2. Always require an initial value
3. Default the initial value to grb::monoid_traits<Fn, T>::identity() when the binary operator passed in is a monoid.

At some point we should implement this full algorithm, but for now I am leaving this unimplemented as it is fairly trivial to use std::reduce at the moment, as shown below.

grb::matrix<float> x = ...;
grb::values_view view(x);
float sum = std::reduce(view.begin(), view.end(), float(0), grb::plus{});