Add `TimeSpans.translate` implementation?

[ericphanson]

function TimeSpans.translate(span::AlignedSpan, t::TimePeriod)
    n = AlignedSpans.n_samples(span.sample_rate, abs(t)) * sign(t)
    return AlignedSpan(span.sample_rate, span.first_index + n, span.last_index + n)
end

The fallback works but outputs a TimeSpan, so you may need to round again to get back. This also has the property of always preserving the number of samples contained in the span, which maybe it's possible to get wrong with pathological rounding otherwise.

[ssfrr]

Just hit this also. I had the idea to start with a "template" AlignedSpan and create a bunch of AlignedSpan babies by shifting that template around to load different parts of a signal, but then got confused when the resulting duration was off.