Test and document structured bindings

[kabasset]

Should make record and column reading so much nicer!

[kabasset]

Works like a charm; to be documented and used in examples.

[kabasset]

Add overload Header::parseSeq<T>(Ts...) somehow to enable homogeneous structured bindings, e.g.:

auto [ra, dec] = h.parseSeq<float>("RA", "DEC");