- similar to what std::copy_n and std::fill_n would do, except with
templated loops. This allows compile-time transcribing with loop
unrolling. For example,
vector vec1 = ..., vec2 = ...;
FixedList<scalar, 6> values;
VectorSpaceOps<3>::copy_n(vec1.begin(), values.begin());
VectorSpaceOps<3>::copy_n(vec2.begin(), values.begin(3))
// do something with all of these values
STYLE: make start index of VectorSpaceOps optional
ENH: add clamped begin(int) versions to FixedList as per UList
