Add SONATA-based instantiation from simulation config

[ilkilic]

CircuitSimulation.instantiate_gids() currently requires explicit cell IDs. However, the SONATA simulation config already defines which cells to instantiate via the top-level node_set.

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Proposal

Introduce a SONATA-aware entry point, for example:

sim.instantiate_from_config()

This should:

• read the top-level node_set from the simulation config
• resolve the corresponding cells internally via libsonata / circuit_access
• follow SONATA defaults if no node_set is defined

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Why

• Align with SONATA semantics
• Avoid upfront ID materialization in upstream tools
• Keep instantiate_gids() as the low-level API for explicit IDs

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Note

SONATA target resolution already exists internally, e.g. for stimuli, but not for cell instantiation.