Patricia Cornwell showed up to her first meeting at the Richmond, Virginia, medical examiner’s office in the summer of 1984 carrying what appeared to be a cane. The woman who would one day invent the forensic thriller was just 28 years old and still unpublished, living in a cramped seminary apartment with her then-husband while she tried to write mystery novels that no one wanted to buy. A friend had arranged an introduction to deputy chief medical examiner Marcella Fierro, and Cornwell was determined to make an impression. “The secretary saw me walking in and said, ‘What do you have a cane for?’” Cornwell told the Post in an exclusive interview. “And I said, ‘Oh, this isn’t just any cane.’” In a new memoir, crime author Patricia Cornwell reveals the great lengths she has gone to in order to research her books —  including the Scarpetta novel series and “Portrait of a Killer,” a non-fiction book about Jack the Ripper. Patrick Ecclesine.…