By Andrea Shalal WASHINGTON (Reuters) – Switzerland-based Glencore, China and Chad’s other creditors have reached an agreement in principle on a debt treatment plan for the African country that should be finalized in coming days, a source close to the negotiations said on Thursday. Chad’s bilateral creditors – China, France, India and Saudi Arabia – agreed several weeks ago that the country did not require debt relief at the moment given higher oil prices, but that they would reconvene and offer Chad help if needed. Glencore , a major private creditor, agreed late in October or early November …