Remove extraneous dss calls

Author: charleskawczynski

src/solvers/imex_ark.jl

@@ -96,7 +96,7 @@ function step_u!(integrator, cache::IMEXARKCache)
             end
         end
 
-        dss!(U, p, t_exp)
+        i ≠ 1 && dss!(U, p, t_exp)
 
         if !(!isnothing(T_imp!) && !iszero(a_imp[i, i])) # Implicit solve
             post_explicit!(U, p, t_imp)

src/solvers/imex_ssprk.jl

@@ -94,7 +94,7 @@ function step_u!(integrator, cache::IMEXSSPRKCache)
             @. U_exp = (1 - β[i - 1]) * u + β[i - 1] * U_exp
         end
 
-        dss!(U_exp, p, t_exp)
+        i ≠ 1 && dss!(U_exp, p, t_exp)
 
         @. U = U_exp
         if !isnothing(T_imp!) # Update based on implicit tendencies from previous stages