[Post V16] create a System to ReactionSystem converter

Given we now support much of the key System features, it would be nice to provide a simple way to convert `System`s to `ReactionSystem`s for use in compositional modeling (instead of having to make dedicated dispatches to combine `System`s into `ReactionSystem`s). The converter should warn if there are `System` fields that are meaningfully populated and can't be brought over to a `ReactionSystem` because we don't support them yet. For base models though there shouldn't really be issues at this point.