Google continues to chip away at the traditional mobile operator business model in the US, this time by extending its Project Fi multi-network MVNO service to data-only devices such as tablets, including iPads. Using a free data-only SIM card ordered from their Project Fi online account page, customers can access cellular networks at a flat rate of $10 per gigabyte. The option is only available, for now at least, to those who already subscribe to the main Fi service, which charges $20 a month for unlimited domestic talk and text, unlimited international texts, WiFi tethering to use the phone as a hotspot, and cellular access in more than 120 countries. The tablet cannot be used as a hotspot. Up to…