AMHERST — From pills to powders, from grapefruit diets to weight-loss systems based on blood type, there are hundreds of diet programs on the market. Amid what can be a confusing sea of choices, selecting the right one can be overwhelming. In Amherst, Jeanette Wilburn and Stephanie Friedrick, a registered nurse, teamed up in October 2011 to open Amherst Ideal Weight Loss at 379 College St., where they offer what they say is an effective and healthy approach to shedding unwant