PyLops/pylops-mpi: v0.4.0

Added pylops_mpi.Distributed.DistributedMixIn class with communicator-agnostic calls to communication methods. Added pylops_mpi.utils._mpi with implementations of MPI communication methods. Added kind="summa" implementation in pylops_mpi.basicoperators.MPIMatrixMult operator. Added kind paramter to all operators in pylops_mpi.basicoperators.MPILaplacian Added cp.cuda.Device().synchronize() before any MPI call when using Cuda-Aware MPI. Modified pylops_mpi.utils._nccl.initialize_nccl_comm to handle nodes with more GPUs than ranks. Fixed bug in pylops_mpi.DistributedArray.__neg__ by explicitely passing base_comm_nccl during internal creation of distributed array .