scale step sizes relative to reference step

[arun3688]

Purpose

Scale initial, minimum, and maximum step sizes by refStep to ensure safe and consistent variable-step solver behavior. Sets defaults:

   refStep = commandLine > defaultExperiment.StepSize > default Value
   initialStepSize = 1e-3 × refStep
   minimumStepSize = 1e-12 × refStep
   maximumStepSize = refStep

This prevents tiny steps at t=0 and improves solver stability.

[arun3688]

@rruusu see if this looks fine