Simplify imex ark

[charleskawczynski]

I think we could simplify imex ark. An attempt was made in #212, but there were some issues (also, #225 is a blocker to all of the annotations).

But the basic idea is nice: re-write the code such that we have:

for i in 1:N_implicit
 # do explicit
 # do implicit
end

 # do final explicit

It would be nice to have a clearer separation of the stages. Also, right now, there are a few things that appear to happen near the end, but due to empty ranges doing nothing (for i in 1:0; # work; end does no work ), code at the end of stage_update! is what actually gets executed first, which is confusing. The hard-coded ARS343 is a really nice piece of code to compare against.