Combine parsed presentation linkbase with facts from instance document

[manusimidt]

As mentioned in #20 and #21 it would be great if the parser would automatically map facts from the instance document to the presentation linkbase.
Currently the parser parses the presentation linkbase but only stores the xml_id of the concept that the locator in the presentation linkbase is referencing. It would be nice if the parser would provide a function that returns a hierachical structure that is arranged according to the presentation linkbase and that also stores the facts.