Add simulation speedup support.
Clamp 1M means no clamp at all and integral component lead PID astray.

Set realistic clamp (below max control value),
Allow ReflowDevice to query current process time, for simulation usage.
Factor out diff() function and refactor other diff_*() to use it.
Add ReflowDevice based on simulation model (delay_model.py).
Make main app executable.
Switch default ReflowDevice plugin to simulation model.
Add .gitignore.