Enable Replacing Components at Any Composite Level

[lrennels]

We want to make it easier for users to replace components in composite models, a concrete example being replace the climate component of a composite version of FUND with composite FAIR as presented below in some rough pseudocode.

In general the current problem is that anything below the top layer of a composite component is more or less immutable, which creates some problems when we want to replace/edit things etc. without rebuilding everything.

add MimiFUND # let's say that the default model pulled in by get_model() is a composite model with three top-level components `:climate`, `:emissions`, and `:damages`, but it's possible that we'd be wanting replace something deeper
add MimiFAIR
import MimiFAIR: `FAIR_climate` # we import the MimiFAIR `FAIR_climate` component which is a composite component

m = MimiFUND.get_model()
replace_comp!(m, :climate, :FAIR_climate)
# reconnect things
run(m)