StudentMaterialSelector is now showing the data provided by ViewModel.
Merge pull request #247 from henningmu/issue#192-real_data_in_StudentMaterialSelector

StudentMaterialSelector is now showing the data provided by ViewModel.
Added doDeleteTeachingMaterial method to BookManagementViewModel
Datenstrucktur der noch nicht erhaltenen BorrowedMaterials an die des
RetrunViewModels angepasst.
Merge pull request #249 from davvitt/issue#246-LendingVMDatenstrucktur

LendingViewModel Datenstruckturanpassung
Bug in Entities equals() Methoden behoben
Integrated the new state from lendingVM into StudentMaterialSelector
Merge pull request #248 from Junkern/issue#123

Added doDeleteTeachingMaterial method to BookManagementViewModel
Merge pull request #251 from davvitt/issue#250-equalsBug

Bug in Entities equals() Methoden behoben
LendingView now works. PDF creation throws errors. Talk to @B3nRa
integrated the return view. still get an error in pdf creation.
Opened an issue for this (see issue #252)
Merge pull request #253 from henningmu/issue#195-add_functions_to_lend_return

Issue#195 add functions to lend return
catch error when trying to initialise a persistService a second time
Added check for null of maps
Merge pull request #258 from davherrmann/issue#225

catch error when trying to initialise a persistService a second time
Merge pull request #259 from B3nRa/issue#252

Added check for null of maps
Added form validation for updating/adding teaching materials.