Feature performance improvements

I did some refactoring regarding the PDF exports. Until now, a separate database query inside the jasper report template was used to get the report data. The required IDs (courses, users etc) were passed with a paramater. For the detailled report, the performance was very slow (up to 40 seconds). 
Now, the PDF export is generated using a CSV file as datasource. This file is stored temporarily in the ILIAS temp dir, passed to JasperReport and deleted after the report is generated.

The following commits should be reviewd before merging in dev branch:
- b103706
- fff6ab2

The title, template_file and output_file are different in each PDF report. I think it's not possible to initialize them in the superclass. Those lines were modified because I switched the variable names from CamelCase to underscore.