Add optional comm parameter for custom MPI communicators

[williamjameshandley]

Summary

• Add optional comm parameter to BinMinMPI, BinMinBottomUp, and all interface functions
• Allows users to pass custom MPI communicators instead of always using MPI.COMM_WORLD
• Enables running binminpy on a subset of MPI ranks or integrating into larger MPI applications

Changes

• BinMinMPI.__init__(): Added comm=None parameter, store as self.comm
• BinMinBottomUp.__init__(): Added comm=None parameter, store as self.comm
• All interface functions (minimize, differential_evolution, etc.): Added comm=None parameter
• _run_optimizer(): Pass comm through to BinMinMPI constructor

Backward Compatibility

Fully backward compatible - when comm is not specified (or is None), defaults to MPI.COMM_WORLD.

Example Usage

from mpi4py import MPI

# Create a subcommunicator (e.g., first 4 ranks)
world = MPI.COMM_WORLD
subcomm = world.Split(color=0 if world.Get_rank() < 4 else 1)

# Use with binminpy
result = binminpy.minimize(
    target_function,
    binning_tuples,
    parallelization="mpi",
    comm=subcomm,  # Use custom communicator
)

Test plan

• Added tests/test_custom_comm.py - Tests BinMinMPI with explicit COMM_WORLD and subcommunicators
• Added tests/test_custom_comm_bottomup.py - Tests BinMinBottomUp with explicit COMM_WORLD
• Run with mpiexec -np 4 python tests/test_custom_comm.py
• Run with mpiexec -np 4 python tests/test_custom_comm_bottomup.py

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