Implement dynamic weight formula in Advanced mode

[wbclark]

This provides the option to use dynamic mask weight to more smoothly blend the conditioning; for example, you can set the weight as =exp(-((x-0.5)**2 + (y-0.5)**2) / (2 * 0.15**2)) to give it a Gaussian distribution (bell durve) that falls off smoothly as it moves away from the center of the image, or use two regions with respective weights =y and =(1-y) to gradually transition between the prompts as you move from top to bottom along the image.