Bad error metric in some 1:1 function comparison tests
Some tests apply a bad error metric to check whether two functions are equal.
For example this applies here:
const auto absScalarProd = std::abs( err.dotGlobal( ones, level, location ) );
WALBERLA_CHECK_LESS( absScalarProd, eps );
But the test could pass even if the error is large, if it just alternates at each point for instance. That is unlikely, but extremely misleading once it happens. Rather we should use the 2- or inf-norm here.