A longtime pharmaceutical executive who oversaw the launch of Biogen Idec's first multiple sclerosis drug, Avonex, has been named CEO of a privately held Newton biotech. Chiasma Inc., which recently raised $70 million in Series E financing and last week told the Boston Business Journal that it's in the process of looking for a new, larger headquarters in either Newton or Cambridge, today named Mark Leuchtenberger as its president and CEO.