add interface for checking whether the default instance is defined

Would it be possible to add int yac_initialized(); or similar (returning 1 if the default instance was initialized, 0 otherwise) to YAC's C API, following xt_initialized() in YAXT, MPI_Initialized(), PetscInitialized() in PETSc, etc?

I am trying to be careful about initializing libraries PISM uses and sometimes (specifically: in Python scripts) I cannot ensure that our code is executed first and is the only code trying to initialize them.

For MPI, for example, PISM's Python bindings will call MPI_Init() if MPI was not initialized yet and will skip this call if someone imported mpi4py and then PISM (the same for PETSc and petsc4py).

This would also help with finalizing libraries at process termination since I would be able to keep track of whether I initialized it (and therefore am responsible for finalizing).