delegate all comparison methods

[goretkin]

As pointed out in JuliaIntervals/IntervalArithmetic.jl#237 (comment)

the assumption at https://github.com/PainterQubits/Unitful.jl/blob/47d131be92eefae1b49cd02b0a9f9a55b74be97f/src/quantities.jl#L324

<=(x::AbstractQuantity, y::AbstractQuantity) = <(x,y) || x==y

might not hold. Take for instance

using IntervalArithmetic: IntervalArithmetic, interval
using Unitful: Unitful, m

a = interval(0, 0)
b = interval(0, 1)
@assert a <= b
@assert a*m <= b*m  # assertion failure here

and the reason why is:

julia> a <= b
true

julia> a < b
false

julia> a == b
false

It seems worthwhile to delegate all comparison methods, not only isless and <, at
https://github.com/PainterQubits/Unitful.jl/blob/47d131be92eefae1b49cd02b0a9f9a55b74be97f/src/quantities.jl#L263-L275