Allow StaticArray or FieldVector with extra fields

It would be nice if custom defined StaticArrays or FieldVectors could have extra fields which would not form part of the array. Example:

struct Point3D <: FieldVector{3, Float64}
    x::Float64
    y::Float64
    z::Float64
    proj::String # Proj4 or similar string
end

This feature would allow to use arbitrary fields of a type to be exposed as a vector, and then be used in e.g. diff-eq solver.

I don't think this can work with the current similar_type interface. If similar_type would take an instance instead of a type, then that could be used to make an appropriate constructor.

Review the StaticArray and FieldVector definitions and the similar_type interface first. Confirm how custom fields can be excluded from the array representation while remaining available on the type, and verify that the resulting vector can be used by differential-equation solvers.