This just aggregates all TextPosition objects that are part of one line. As not every line break
in the original PDF document has meaning for the structure of the text we usually throw the line
information away and just use it for the processing. Thus this is called a "PreTextLine".
Author:
Benjamin Paassen - bpaassen(at)techfak.uni-bielefeld.de
Copyright (C) 2013, 2014 Raphael Dickfelder, Jan Göpfert, Benjamin Paassen, Andreas Stöckel, licensed under the AGPL v. 3: http://openresearch.cit-ec.de/projects/scie