Better use of pybind11 scoped interpreter

The following discussion from !4569 should be addressed:

• @chleh started a discussion: (+1 comment)

  @bilke, I can see the next problem at the horizon. From the pybind11 docs:

  /** \rst
      Scope guard version of `initialize_interpreter` and `finalize_interpreter`.
      This a move-only guard and only a single instance can exist.
  
      See `initialize_interpreter` for a discussion of its constructor arguments.
  
      .. code-block:: cpp
  
          #include <pybind11/embed.h>
  
          int main() {
              py::scoped_interpreter guard{};
              py::print(Hello, World!);
          } // <-- interpreter shutdown
   \endrst */
  class scoped_interpreter

  We should remove the std::optional<pybind11::scoped_interpreter> _py_scoped_interpreter; member, soon. Yes, I suggested adding it, but maybe it was not a very robust hint. I assume as soon as people start playing around with OGS bindings in their Python scripts, they will face strange errors.

  I guess we need some ref-counted thread-safe global variable taking care of the embedded Python interpreter (shared_ptr and mutex).