Move the walberla bridge to a self-contained git repo

[RudolfWeeber]

No description provided.

[RudolfWeeber]

Make the walberla_bridge (located in src/walberla_bridge) an independent library, i.e.,

• it should live in its own git repo
• and have its own CMake, such that it can be added to other cmae project, and that it knows how to run its own tests

The goal is that the library can be used by other codes than Espresso, e.g., our parterns in MultiXScale.

[jngrad]

Requirements:

• Provide a copy of cmake/unit_test.cmake
• Replace Utils::Vector3d by a class with similar functionality
• Remove Boost dependency (see walberla/walberla#190)
  • Replace boost::optional by std::optional
  • Replace boost::variant by std::variant
  • Replace boost::multi_array by equivalent code
  • unit tests can probably still use Boost

[RudolfWeeber]

The first step is to

• copy the content of src/walberla_bridge into a new git repo outide espresso
  walbela_bridge
• src/utils/include/utils/vector.hpp from Esprsso will also be needed for Vector3d and friends
• Modify the CMakeList.txt sot it can be compiled independently
• This involves copying in the combiler definitoins and walberla importing code form ESpresso's CMakeList.txt. Currently Espresso imports Walberla as a cake subproject. This will have to be done in the smae way for the walberla bridge repo.

Once the Walberla bridge can be built in its own repo independently of ESPRso, we'll sort out using that new repo from Espresso.

[jngrad]

A proof-of-concept can be found in jonscheunemann/walberla_bridge. Thanks @jonscheunemann and Marc Sauter!

Once we're done with the major LB/EK rewrite, we'll be able to use that work to create the bridge as part of the ESPResSo organization. I'll take care of resolving any remaining CMake subtleties with the FetchContent method.