Adding new light profile into globals

Lee and I have been playing with this light profile, and I'd like to propose it as a nice addition to Globals and to the PWM functions.

It ramps up and down with a long tail at the end with a "sigmoid" shape in each direction.  Technically it's the "logistic" function 1/(1+e^-x).  We view it as an alternative to PWMParabola.  As PWMSmoothRamp is to PWMSlope which rounds out the corners of Slope, PWMSigmoid would do the same for PWMParabola.  Gives nice long tails in the morning and evening with a round peak in the middle.  There is no Duration argument in the PWMSigmoid function, similar to the Parabola one.  

You can see what it looks like at [scpwme9](http://forum.reefangel.com/status/chart.aspx?id=AlanM&filter=scpwme9)