Multiplicative and addititive identities

[tomasaschan]

This introduces one(x) and zero(x) for x of type SIQuantity.

The behavior is designed so that the two methods are multiplicative
and additive identities, respectively, i.e.

x * one(x) == x
x + zero(x) == x

This is intended to fix JuliaLang/julia#18 and at least help mitigating JuliaLang/LinearAlgebra.jl#128.

[mbauman]

Should these also be defined for the type? E.g., one(::Type{SIQuantity})?

Similarly useful may be nan.

[tomasaschan]

@mbauman

Should these also be defined for the type? E.g., one(::Type{SIQuantity})?

Probably a good idea. Looking at methods(one) shows, among other things,

one{T}(::Type{UniformScaling{T}}) at linalg/uniformscaling.jl:20
one{T}(J::UniformScaling{T}) at linalg/uniformscaling.jl:21

indicating that if these two are there, we probably have what we need. zero behaves similarly. I'll add these two shortly.

Similarly useful may be nan.

I'm not sure if it makes sense to have units on a NaN value. If a NaN value appears somewhere, something is probably broken, and preserving unit information is (or should be) irrelevant. If there's a use case, it's probably not difficult to add, though, but I think it can wait until someone needs it.

[mbauman]

If a NaN value appears somewhere, something is probably broken

Very good point. I temporarily defined nan for SIUnits at one point as I was debugging a problem… and it turned out to only be necessary because something was indeed broken. Probably better to leave it out until we have a definitive use-case. Thanks for doing this!

[tomasaschan]

There - now it will work with types as well.