Proposal: new types for independent measurements and dependent measurements

We could make Measurement type abstract, and define two concrete types, IndependentMeasurement and DependentMeasurement (the latter may be an abuse of language but gives the idea):

abstract Measurement{T<:AbstractFloat} <: AbstractFloat

immutable IndependentMeasurement{T<:AbstractFloat} <: Measurement{T}
    val::T
    err::T
    tag::Float64
end

immutable DependentMeasurement{T<:AbstractFloat} <: Measurement{T}
    val::T
    err::T
    der::Derivatives{Tuple{T, T, Float64}, T}
end

measurement function would create IndependentMeasurement objects, while result function in src/math.jl would create DependentMeasurement objects. This function could have a specialized method for IndependentMeasurement only arguments, which could be fairly efficient.

Ideally, there would not be user-visible changes, apart from performance improvements.

I'm not sure this proposal will actually improve performance, I'm leaving this as a memo for me open for discussion.

Start by reading the proposed Measurement, IndependentMeasurement, and DependentMeasurement definitions, then inspect src/math.jl and the measurement and result entry points. Determine whether the separation can preserve user-visible behavior and improve performance; the work is done only if the design is agreed and those effects are demonstrated.