WASHINGTON — Diamonds may be forever and a girl’s best friend. They may be the centerpieces of crowns and countless heist films. They may even motivate treasure hunters to plumb the icy grave of the RMS Titanic. But can they revolutionize battlefield sensors ? Raytheon thinks so. The company, a division of RTX, said it won $15 million from the Defense Advanced Research Projects Agency, or DARPA, to improve gallium nitride semiconductors utilizing the gemstones. Used in radars, gallium nitride