Asymmetric inplace operations

[niklas-uhl]

• [ ] gather
  • pass MPI_IN_PLACE on root, sendcount and sendtype ignore, contribution of root is assumed to be in the correct place in the buffer
• [ ] reduce
  • pass MPI_IN_PLACE as send_buf on root, data taken from recv_buf
• [ ] gatherv
  • pass MPI_IN_PLACE on root, sendcount and sendtype ignore, contribution of root is assumed to be in the correct place in the buffer
• [ ] scatter
  • pass MPI_IN_PLACE on root, sendcount and sendtype ignore, root just "keeps" vector to scatter
• [ ] scatterv
  • same as scatter
• [ ] alltoallv
  • scounts, displs, sendtype ignored
  • send and recv count between a processor pair must be the same -> limitation!

Originally posted by @niklas-uhl in https://github.com/kamping-site/kamping/issues/28#issuecomment-1940684517