fix description-reading for logicgraph-parsing

The characters method can be called multiple times in order to retrieve
the full text (like chunks) so we'll need to concatenate the results.
let description for logicgraph default to nil

This is needed for possible updates of description
on subsequent runs of the logicgraph-parsing.
respect newlines when displaying logic description.
Merge pull request #1091 from ontohub/1088-fix_logicgraph_parser

1088 fix logicgraph parser