Add gmin stepping to VCCS/CCCS for convergence aid

[esaruoho]

Summary

• Fixes convergence failures in circuits with controlled sources in positive feedback loops (Getting 'Convergence failed!' with converting optocoupler to Schmitt trigger #172)
• Adds gmin stepping (parallel conductance that gradually increases with subiterations) to both VCCSElm and CCCSElm, following the same well-established pattern used by Diode.java and TransistorElm.java
• The optocoupler Schmitt trigger circuit in Getting 'Convergence failed!' with converting optocoupler to Schmitt trigger #172 fails because the CCCS inside the optocoupler oscillates between two states with no damping mechanism — the solver bounces back and forth for 5000 iterations then gives up
• Uses a slower ramp rate (subIterations/3000) compared to transistors (subIterations/300) to minimize impact on normally-converging circuits

Test plan

• Build the optocoupler Schmitt trigger circuit from issue Getting 'Convergence failed!' with converting optocoupler to Schmitt trigger #172 → verify it converges
• Test standard VCCS and CCCS circuits → verify no regression
• Test optocoupler in normal (non-feedback) configuration → verify correct behavior
• Test circuits with multiple controlled sources → verify convergence

Fixes #172

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