Let users define fallback eos_constrain_step

[stepanzh]

Currently, eos_constrain_step fallbacks to unconstrained condition.

CubicEoS.jl/src/vt_split/vt_split.jl

Lines 64 to 76 in bcb215f

	function vt_split(
	mix::AbstractEoSMixture{T},
	nmol::AbstractVector,
	volume::Real,
	RT::Real,
	concentration::AbstractVector,
	StateVariables::Type{<:AbstractVTSplitState};
	tol::Real=1024*eps(T),
	chemtol::Real=tol,
	presstol::Real=tol,
	maxiter::Integer=100,
	eos_constrain_step::Function=unconstrained_eos_step,
	) where {T}

I guess, it's better to let users define default constrain step function.

It may be defined as eos_constrain_step_factory(::Eos, ::Variables) -> eos_constrain_step(::Vars, x, d) or similar.

In CubicEoS interface, we define the factory producing unconstrain step, this is CubicEoS's default. But, user can define it's own default.