change error handling
add test for budget item save/load
fix gcc compilation error
refactor smart ptr usage
change error handling in serializers

throw std::runtime_error if file is not opend
Distributed items tools refactoring
t_Holder typedef added to main item classes
implement items creation integration within Holder & Factory
modify file-saving test for Budget
minor changes
improve file-saving tests
Merge branch 'feature/save_items_to_disk' into develop