remove SessionModel/Item from SampleModel and all related items
Huge refactoring of SampleModel & related classes
- Remove any dependency to SessionModel/SessionItem from all related classes
- Add serialization, which is already prepared for better version handling (backwards compatiblilty)
- Adapt unit tests
There are still a few spots to be implemented (mainly regarding serialization, tuning tree creation). To make this MR not even bigger, they will be done in upcoming MRs