Deprecate dune_add_pybind11_submodule

The following discussion from !1461 (merged) should be addressed:

• @simon.praetorius started a discussion: (+1 comment)

  This MR extracts the functionality from dune_add_pybind11_submodule and creates an object library with all the flags manually. This seems to work, as tests have confirmed. The question is whether we prefer to use a cmake function for this, that combines all the individual calls, or do it by hand.

  • Manual creating an object library gives us the full flexibility. In total it is 1-2 lines more code, but safes the 50lines for the cmake function.
  • A function could be easier changed for several modules and we could reuse existing code. The dune_add_pybind11_submodule function might be a bit misleading for this task, since it is not related to pybind11 and does not create a submodule of something (whatever this is). It is essentially just a more fancy way to writing target_source with extra flags passed to the sources.