Make error norm calculation optional

src/callbacks_step/analysis.jl

@@ -23,6 +23,13 @@ Additional errors can be computed, e.g. by passing
 `extra_analysis_errors = (:l2_error_primitive, :linf_error_primitive)`
 or `extra_analysis_errors = (:conservation_error,)`.
 
+If you want to omit the computation (to safe compute-time) of the [`default_analysis_errors`](@ref), specify
+`analysis_errors = Symbol[]`.
+Note: `default_analysis_errors` are `:l2_error`, `:linf_error` for all equations.
+If you want to compute `extra_analysis_errors` such as `:conservation_error` solely, i.e., 
+without `:l2_error, :linf_error` you need to specify 
+`analysis_errors = [:conservation_error]` instead of `extra_analysis_errors = [:conservation_error]`.
+
 Further scalar functions `func` in `extra_analysis_integrals` are applied to the numerical
 solution and integrated over the computational domain. Some examples for this are
 [`entropy`](@ref), [`energy_kinetic`](@ref), [`energy_internal`](@ref), and [`energy_total`](@ref).
@@ -332,7 +339,8 @@ function (analysis_callback::AnalysisCallback)(u_ode, du_ode, integrator, semi)
         @notimeit timer() integrator.f(du_ode, u_ode, semi, t)
         u = wrap_array(u_ode, mesh, equations, solver, cache)
         du = wrap_array(du_ode, mesh, equations, solver, cache)
-        l2_error, linf_error = analysis_callback(io, du, u, u_ode, t, semi)
+        # Compute l2_error, linf_error
+        analysis_callback(io, du, u, u_ode, t, semi)
 
         mpi_println("─"^100)
         mpi_println()
@@ -354,8 +362,7 @@ function (analysis_callback::AnalysisCallback)(u_ode, du_ode, integrator, semi)
     analysis_callback.start_time_last_analysis = time_ns()
     analysis_callback.ncalls_rhs_last_analysis = ncalls(semi.performance_counter)
 
-    # Return errors for EOC analysis
-    return l2_error, linf_error
+    return nothing
 end
 
 # This method is just called internally from `(analysis_callback::AnalysisCallback)(integrator)`
@@ -382,9 +389,7 @@ function (analysis_callback::AnalysisCallback)(io, du, u, u_ode, t, semi)
         l2_error, linf_error = calc_error_norms(u_ode, t, analyzer, semi,
                                                 cache_analysis)
     else
-        # Names `l2_error`, `linf_error` need to be defined
-        l2_error = nothing
-        linf_error = nothing
+        return nothing
     end
 
     if mpi_isroot()
@@ -484,7 +489,7 @@ function (analysis_callback::AnalysisCallback)(io, du, u, u_ode, t, semi)
     # additional integrals
     analyze_integrals(analysis_integrals, io, du, u, t, semi)
 
-    return l2_error, linf_error
+    return nothing
 end
 
 # Print level information only if AMR is enabled