By calling `super.get_pdf` in print_document we can encounter trouble with MRO resolution
that prevent custom report parser (e.g. override of `get_pdf`) to be called.
The fix consist of not calling `super` and prevent multiple call to 'printer.print_document'
Instead, a notification is displayed to the user.
When report.get_pdf() is called on a report that must be printer,
it will print the report *and* returns the pdf, thus code that
calls directly report.get_pdf() will print the pdf on the printer
as expected.
Fixes#16
