added sequence, coding sequence, and UTR properties to Transcript objects



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I literally meant making this part of the docstring for Transcript. PEP8 says block comments should be `#`, which I agree with: "Block comments generally apply to some (or all) code that follows them, and are indented to the same level as that code. Each line of a block comment starts with a # and a single space (unless it is indented text inside the comment)."