This modules simulates a Solar panel controller with a lead-acid battery, solar panel,
and voltage and current sensors. The module has three sensor pins and two 
control pins. 

The three sensor pins are

    Vsol - Solar panel Voltage
    Asol - Solar panel Current
    Vbat - Battery voltage 

The two control pins are

    PWM - Pulse with modulated signal to controler
    OK  - PWM signal is used (if low PWM is don't care)

Attributes are as follows

Aoffset - Voltage on Asol when current is zero
Ascale  - Volts/Amp scale factor for Asol
BDOC    - Battery degree of charge (determines battery voltage)
VBscale - Vbat (volts) = battery voltage * VBscale
VSscale - Vsol (volts) = solar panel voltage * VScale
inductor - controller inductor in Henrys

The module simulates a 12 solar panel with the following specs

open circuit voltage 21.6V
Short circuit current 1.27A
Max power point 17.3V 1.16A

The simulated panel VI is an approximation and is OK for the current purpose
but should not be taken a correct for design purposes.

The battery uses a simple model for a 12V lead-acid (or SLA) battery with a
20 mah capacity and a 1.5 ohm internal resistance.  The small battery
capacity is to allow the battery to quickly change state.
