Added Builder Pattern to PDFStudentList.
Added two new attributes:
 - returnList
 - lendingList
Added pdf test for dunning.
Removed public constructors of PDFStudentList
Removed old PDFStudentList creation in other classes.
Removed unnecessary null-check.
Added returnList and lendingList to addContent
PDFClassList uses Map<Grade, Map<TeachingMaterial, Integer>> to create
the class pdf.

Changed PDFTest to test the new PDFClassList implementation.
Adapted GradeTest for new PDFGrade implementation with map.
Removed unused code.
Implemented test for multiple class-pdf creations
Class list pdf contains grade suffix now.
Removed unnecessary constructor in PDFClassList.
Added first version of PDFDunning class
Merge pull request #210 from B3nRa/issue#186

Issue#186