Remove BaseModel since it breaks equals/hashCode contract

[Weltraumschaf]

Problem of equals and inheritance:
As described in this[1] blog post the Object#equals() requires that it fulfills the Liskov Substitution Principle (LSP). Our implemenation breaks this contract. Even worse, it didn't work at all as descibed in issue 23[2].

Since the BaseModel class does not have any properties, we can remove it.

This patch removes this obsolete class. Also it makes all model classes final and all properties private. This is an implicit requirement of the contract of hashCode() to avoid memory leaks in collections. (See linked blog post on Artima for more details.)

Actually objects must be immutable -- all fields final -- to guaruantee a stable equal/hashCode behaviour for the whole lifetime of the objects. But this must be further investigated, if this is possible. For now we ignore this warning in the test code.

1: https://www.artima.com/articles/how-to-write-an-equality-method-in-java
2: #23

[Weltraumschaf]

I suggest reviewing by commits instead of files bc I did a reformatting of all classes in one commit.

[J12934]

looks generally good added a couple of comments here to todo question in case that helps give some context

[sonarqubecloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

6 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[J12934]

which commits here are new since the last review?
don't want to go through all of them again and with the force pushes & rebases on this branch the dates are wrong and the github ui doesn't show me where to start again.