Fix MPI pre-cell handling, node set resolution, and post_gid fallback

[ilkilic]

This PR fixes three issues in circuit instantiation:

• Fixes MPI pre-cell handling for connections when presynaptic populations are not locally instantiated on a rank.
• Fixes composite multi-population node set resolution for alias node sets (e.g. node sets defined as lists of other node sets).
• Restores standalone synapse creation compatibility by allowing post_gid to fall back to cell_id.id when not explicitly provided.

[codecov]

Codecov Report

❌ Patch coverage is 95.27559% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tests/test_circuit_simulation_mpi.py	95.23%	3 Missing ⚠️
...ab/circuit/circuit_access/sonata_circuit_access.py	87.50%	2 Missing ⚠️
bluecellulab/circuit_simulation.py	93.33%	1 Missing ⚠️

Files with missing lines	Coverage Δ
bluecellulab/synapse/synapse_types.py	83.02% <100.00%> (ø)
tests/test_circuit/test_circuit_access.py	100.00% <100.00%> (ø)
tests/test_synapse/test_synapse_factory.py	100.00% <100.00%> (ø)
bluecellulab/circuit_simulation.py	84.63% <93.33%> (+0.39%)	⬆️
...ab/circuit/circuit_access/sonata_circuit_access.py	95.81% <87.50%> (-0.76%)	⬇️
tests/test_circuit_simulation_mpi.py	97.43% <95.23%> (-1.05%)	⬇️

... and 1 file with indirect coverage changes

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