Alias variables

When there are physical conversions between components, we often have things like

x.comp1.out = fun(x.comp2.in)

However, when solving differential equations, we must have the same number of states as the number of equations, so we cannot actually introduce the new x.comp2.in variable without adding the trivial algebraic equation

du[i] = x.comp1.out - fun(x.comp2.in)

I wonder if we can add some kind of mechanism such that x.comp2.in directly returns fun(x.comp1.out).

Possible syntax:

x = ComponentArray(
    comp1 = ComponentArray(out=1),
    comp2 = ComponentArray(in=AliasVar(UpperLevel().comp1.out, fun)),
)

where UpperLevel signifies that comp2 array must keep a reference of the upper level ComponentArray x.

The issue names no files, tests, or entry points, so start by examining the existing ComponentArray construction and indexing behavior. Evaluate whether the proposed AliasVar and UpperLevel mechanism fits that design. Done means a defined interface and behavior for alias variables, including how references and conversions are handled.