A fatal house fire is under investigation in Somerset County, the prosecutor’s office said. The fire happened on Cliff Street in Somerville early Sunday morning, officials said. It’s unclear how many fatalities there are, or if there were any other injuries. The Somerset County Prosecutor’s Office Major Crimes Unit and the Somerset County Fire Investigation Unit, Somerville Police Department and Somerville Fire Officials were on scene this morning, officials said. Details about the fire remain limited as the investigation is ongoing, officials said. Katie Kausch may be reached at kkausch@njadv…