In practice, this means that whenever an author is not part of a
collection of periods he will not be printed out.
This cleans up the output and also speeds up analysis.
The output was simply wrong and was outputting extra "<authors>" tags all
over the place. Also, whenever there was very little work, this was not
included in the timeline.
The Outputable class is now the base class of all the classes that want to
output formatted text. This is more object oriented and cleaner solution
compared to the previous implementation.
