A 26-year-old woman was found murdered in her Chelsea apartment by her sister last night, and the murder victim's boyfriend--and the murder suspect, according to police-- killed himself a few hours after the discovery. Margaux Powers's family had been worried they didn't hear from her for a few days, so her sister Dana went to Margaux's apartment at 235 West 22nd Street on Friday night. Doorman Constantine Ionescu accompanied Dana to the apartment, and......