WIP change lb density to delta_density in the generated kernels

[itischler]

Description of changes:

• change to a zero centered lb kernel to hopefully increase precision

[espresso-ci]

Your pull request does not meet our code formatting rules. Specifically, I suggest you make the following changes:

diff --git a/maintainer/walberla_kernels/generate_lb_kernels.py b/maintainer/walberla_kernels/generate_lb_kernels.py
index 000cd7be9..af49cded5 100644
--- a/maintainer/walberla_kernels/generate_lb_kernels.py
+++ b/maintainer/walberla_kernels/generate_lb_kernels.py
@@ -52,7 +52,7 @@ else:
     target = ps.Target.CPU
 
 # Make sure we have the correct versions of the required dependencies
-#for module, requirement in [(ps, "==1.2"), (lbmpy, "==1.2")]:
+# for module, requirement in [(ps, "==1.2"), (lbmpy, "==1.2")]:
 #    assert pkg_resources.packaging.specifiers.SpecifierSet(requirement).contains(module.__version__), \
 #        f"{module.__name__} version {module.__version__} doesn't match requirement {requirement}"
 
diff --git a/src/walberla_bridge/tests/kernels_unit_tests.cpp b/src/walberla_bridge/tests/kernels_unit_tests.cpp
index c9143da79..f0289b37b 100644
--- a/src/walberla_bridge/tests/kernels_unit_tests.cpp
+++ b/src/walberla_bridge/tests/kernels_unit_tests.cpp
@@ -172,7 +172,8 @@ BOOST_AUTO_TEST_CASE(macroscopic_accessor_equilibrium_distribution) {
     }
   }
   {
-    auto const ref_d = delta_rho_d * 1. / 36. + rho_d * (5. / 12. * x * x + 2. / 12. * x);
+    auto const ref_d =
+        delta_rho_d * 1. / 36. + rho_d * (5. / 12. * x * x + 2. / 12. * x);
     auto const ref_f = static_cast<float>(ref_d);
     for (auto const direction : {Direction::NE, Direction::TN, Direction::TE}) {
       auto const pop_f = EquilibriumDistribution::get(direction, u_f, rho_f);
@@ -182,7 +183,8 @@ BOOST_AUTO_TEST_CASE(macroscopic_accessor_equilibrium_distribution) {
     }
   }
   {
-    auto const ref_d = delta_rho_d * 1. / 36. + rho_d * (5. / 12. * x * x - 2. / 12. * x);
+    auto const ref_d =
+        delta_rho_d * 1. / 36. + rho_d * (5. / 12. * x * x - 2. / 12. * x);
     auto const ref_f = static_cast<float>(ref_d);
     for (auto const direction : {Direction::SW, Direction::BS, Direction::BW}) {
       auto const pop_f = EquilibriumDistribution::get(direction, u_f, rho_f);

To apply these changes, please do one of the following:

• You can download a patch with my suggested changes here, inspect it and make changes manually.
• You can directly apply it to your repository by running curl https://gitlab.icp.uni-stuttgart.de/espressomd/espresso/-/jobs/350493/artifacts/raw/style.patch | git apply -.
• You can run maintainer/CI/fix_style.sh to automatically fix your coding style. This is the same command that I have executed to generate the patch above, but it requires certain tools to be installed on your computer.

You can run gitlab-runner exec docker style afterwards to check if your changes worked out properly.

Please note that there are often multiple ways to correctly format code. As I am just a robot, I sometimes fail to identify the most aesthetically pleasing way. So please look over my suggested changes and adapt them where the style does not make sense.

[espresso-ci]

Your pull request does not meet our code style rules. Pylint summary:

************* Module generate_lb_kernels
maintainer/walberla_kernels/generate_lb_kernels.py:21:0: W0611: Unused import pkg_resources (unused-import)
************* Module walberla_lbm_generation
maintainer/walberla_kernels/walberla_lbm_generation.py:150:4: W0612: Unused variable 'delta_rho_sym' (unused-variable)

You can generate these warnings with maintainer/CI/fix_style.sh. This is the same command that I have executed to generate the log above.