Make EulerEq2d run with Kelvin Helmholtz test

• fixed dg weak form computation in 2d for special case when one component of the flux is zero
• fixed/implemented flux and ldg flux equations for 2d
• converted all max calls to absmax calls in equations.jl
• extracted numerical utilities from utils.jl into numutils.jl

The IntegrationTests/refs/euler2d_kelvin_helmholtz_entropyav.toml test now runs with polynomial orders n=1 (up to n=4; but not fully tested). Unfortunately, atm this runs pretty slow, so it is not yet added as ref test, but goal is to make it one in the future.

src/dg1d.jl

@@ -34,9 +34,6 @@ export # parameter handling
        query, splitallext, trim_keys, match_keys, prepend_keys,
        # type utils
        concretetypes,
-       # numeric utilities
-       periodize, periodic_index, # deprecated
-       estimate_convergence_rate,
        # parameter files and file system operations
        mk_outputdir, dirname_path, flatten, get_parameter_file, get_parameter_file_index,
        select_parameter_files, gather_output_directories, gather_parameter_files,
@@ -44,10 +41,18 @@ export # parameter handling
        find_duplicated_parameter_files,
        run_projects, analyze_projects,
        TODO, @TODO, @stringify_names,
-       indent,
-       absmax
+       indent
 include("utils.jl")
 
+
+export LLF,
+       absmax,
+       periodize,
+       estimate_convergence_rate,
+       # deprecated
+       periodic_index
+include("numutils.jl")
+
 export Output, read_variable, read_variable!, read_mesh
 include("output.jl")