Updated electrical system.

Updated electrical system.

These changes represent some attempts to bandaid and patch a hopelessly
flawed system to impliment basic battery charging/discharging as well as
provide the ability to model ammeter gauges and draw current from multiple
sources (like load balancing multiple alternators in a multi-engine aircraft.)
The system design forces all these things to be horrible hacks or depend
on extremely subtle system side effects and call ordering so they may or9
may not work to one degree or another.

As mentioned in the mailing list, my recommendation is to move away from
using this system and instead build a procedural electrical system using
nasal.  Sometime in the future we hopefully can impliment a better conceived
data driven electrical system model.