- Adapted `pylops_mpi.optimization.cg` and `pylops_mpi.optimization.cgls` to handle `pylops_mpi.StackedDistributedArray`.
- Added `pylops_mpi.MPIGradient` operator.
- Added `pylops_mpi.MPIStackedLinearOperator`, `pylops_mpi.MPIStackedBlockDiag`, and `pylops_mpi.MPIStackedVStack` operators.
- Added `pylops_mpi.StackedDistributedArray`.