Fix PR #70 and Review Remaining PRs

[jrfaeder]

Fixed Pull Request #70:

1. Resolved a compilation error in src/NFcore/system.cpp by aligning the System::addConcentration signature with its header declaration.
2. Fixed a logic error in the new stepTo caching mechanism. By ensuring both sim() and stepTo() use random_open() for stochastic waiting-time draws and correctly advancing current_time to the chunk boundary, deterministic reproducibility is now maintained across all simulation paths.
3. Verified the fix with new regression tests (test_step_to_chunking_matches_continuous_run) and the full validation suite (all 58 tests passed).

Review of remaining Pull Requests:

• PR fix: avoid nauty unistd.h include under MSVC #71: Fixes an MSVC build issue by gating unistd.h. Highly recommended for cross-platform compatibility.
• PR Expose System::getNumOfObsForOutput / getObsForOutput for in-process embedders #72: Exposes necessary public accessors on the System class for library consumers. Low risk, high utility for embedders.
• PR Add BioNetGen-compatible tfun support #69: Implements comprehensive BioNetGen-compatible tfun support. This is a high-value feature with solid documentation and regression coverage.
• General Observation: There are over 30 older unmerged PRs. PR Performance, correctness, eBNGL state-change expansion, MPI/tooling hardening, and new unit tests #64 was recently merged, but others like Add support for fixed species (clamped populations) #60 (Fixed Species support) appear to be significant features that may require updated review against the current master.

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PR created automatically by Jules for task 1616908949059018078 started by @jrfaeder

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