Proposal for rounding with numerical `Quantity` resolution: round(precision::Quantity, x::quantity)

[awbertulli]

Hi, thank you for your work on Unitful.jl. I was looking for a way to round a Quantity up to a certain resolution, and i discovered there isn't method defined. We can round up to a certain unit, but not a measure. Meaning, we can round a meter up to a cm, but not up to the nearest 5cm measure. May i suggest this:

function round(δ::Quantity, x::Quantity; kwargs...)
    if ustrip(δ) == 0
        round(unit(δ), x; kwargs...)
    else
        round(x / δ, RoundNearest; kwargs...) * δ
    end
end

julia> round(5u"cm", 1u"m")
100.0 cm

julia> round(5u"cm", 0.94u"m")
95.0 cm

EDIT: this is similar, but different, to #326