Compute and add tendencies by default

[charleskawczynski]

One pattern that we're currently using is:

# ClimaODEFunction(;
T_exp! = nothing
)

...

if !isnothing(T_exp!)
     T_exp!(U, p, t)
end
...
if !isnothing(T_exp!)
     @. U_exp += dt * T_exp
end

This really complicates the code. I think the algorithm will be much easier if we by default initialize T_exp = zeros and have T_exp! = (u,p,t) -> nothing. The optimization as it is, is really only benefiting toy examples where, for example, T_exp (or T_lim) don't exist. If they're toy examples, adding these zeros (@. U_exp += dt * T_exp) is should not be an issue. Basically, I think we shouldn't be optimizing for toy examples, and write the code in a simpler way for the real world example, but also allow it to work for toy examples.