Load balancing (proposed in #3662) requires a library
librepa https://github.com/SC-SGS/repa
which in turn needs
kdpart https://github.com/hirschsn/kdpart

These dependencies need to be integrated into ES's build system via CMake fetch_content

Notes:

• See the CMake logic in the Walberla branch for an example on using fetch_content in the ES build system
• Until the PR is merged, Fix warnings from clang 10 SC-SGS/repa#50 needs to be used.
• For the time being, the boost version check in librepa's CMakeList.txt can be disabled. The relevant bug is not reproducible on the ICP systems, but the isseue will go away with Ubuntu 22.04, whatever. (Tests fail for boost >= 1.69.0 SC-SGS/repa#16). According to /usr/share/doc/libboost-mpi-1.71-dev/Chanelog.Debian, soem bugfixes have been backported. It is not clear if the relevant is among them. We'll have to make a final decision before releaseing 4.2, in case load-balaning makes it in there.
• kdpart is built via Makefile. This shuold still work with CMake's fetchcontent https://stackoverflow.com/questions/58644504/cmakes-fetchcontent-with-makefile-built-dependency
• We should try to make sure, kdpart is built with the same compiler like the rest of ES and librepa
• Cmake command for librepa cmake . -DKDPART_LIBRARIES=$HOME/kdpart/kdpart/libkdpart.so -DKDPART_INCLUDE_DIR=$HOME/kdpart -DWITH_P4EST=off -DWITH_PARMETIS=off
• On the ICP machines, no additional dependencies were rewuitrf. Yo my unfrtdysnfinh, kdpart and librepa should also build in our CI docker containers. This needs to be checked, in the end.