[Post V16] create a System to ReactionSystem converter

[isaacsas]

Given we now support much of the key System features, it would be nice to provide a simple way to convert Systems to ReactionSystems for use in compositional modeling (instead of having to make dedicated dispatches to combine Systems into ReactionSystems). 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.