[WIP] Add checkpoint/resume functionality to EMD solver

[nathanneike]

Implements pause and resume capabilities for the EMD (Earth Mover's Distance) solver, allowing long-running optimizations to be interrupted and continued from their exact state.

Changes:

• Modified EMD_wrap() signature to accept checkpoint parameters for saving and restoring complete internal solver state (flow, potentials, tree structure)
• Added saveCheckpoint(), restoreCheckpoint(), and runFromCheckpoint() methods to NetworkSimplexSimple class in network_simplex_simple.h
• Extended emd_c() Cython wrapper to handle checkpoint dictionaries with 12 fields (10 arrays + 2 scalar arc counts)
• Added 'checkpoint' and 'return_checkpoint' parameters to emd() Python API
• Includes search_arc_num and all_arc_num scalars in checkpoint to preserve initialization state required by start() method

Tests:

• Added test_emd_checkpoint() for basic save/resume functionality
• Added test_emd_checkpoint_multiple() for multiple pause/resume cycles
• Added test_emd_checkpoint_structure() to verify checkpoint field integrity

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.