Adjustable turbine auto-balance thresholds

Add the ability to manually set the low-water mark (when turbines start up) and possibly also high-water mark (when turbines shut down) for auto-balanced power turbines.
Justification: Long(ish) steam pipes can lead to spin-up lag and brownouts when the power generators are pushed near capacity at the transition point from off to on. Especially noticeable with combo high-pressure/low-pressure setups as low-pressure steam wends its way to from the HPT exhaust to the LPT input. Setting the low-water mark higher would trade efficiency for reliability.