Allowing fixed parameters (parameters that don't vary during optimization)

[atrophiedbrain]

When performing parameter estimation of a system of ODEs, one may need to pass in data that is necessary as part of the ODEFunction calculation but is not actually a parameter that needs to be optimized.

At the moment, one can declare these values as const and reference them in the ODEFunction or pack them into a tuple that is passed to the ODEFunction.

I note here in DifferentialEquations.jl, boundary information is passed at the optimizer level.

In an attempt to allow the p tuple passed to the ODEFunction to be divided up, I propose the following:

Could another tuple be passed at the optimizer level with boolean values to denote whether the corresponding entry in the packed p tuple should vary?