This allows books in a series category to be ordered by their series number if available.
Putting this up for review because the code is a bit hacky and I'm not sure if it cannot be done better than this.
I briefly also played with using regexps to get the series number from a file name, but I couldn't find a foolproof way of getting the chapter number instead of any other digit, so now chapters without seriesnumber are set at 0 and thus ordered by title.