Wrapper for MPI_Intercomm_create

Dear MPI.jl team,

I have not found any wrapper for `MPI_Intercomm_create` nor in the documentation nor by grep-ing into de source code.

Is there a specific reason that prevents to provide a julia function for `MPI_Intercomm_create`? or it is simply because nobody has needed/implemented it so far?

Thanks for the nice package!