Generate easier POs for non-trivial diverges clauses

Related Issue

This pull request fixes #29.

Intended Change

A non-trivial (i.e, neither true nor false) diverges clause results in two contracts to be verified. Previously, there was one where the value of the clause (here div) is not true and the method therefore terminates, and one where the method does not terminate.

E.g., the two POs would look like pre & !div -> <m()>post and pre -> [m()] post. This resulted in more complex proofs, because the case where div is false had to be considered as well for the second PO.

This PR changes the POs to

pre & !div -> <m()>post and
pre & div -> [m()] post

Hence, we have a proper cut.

Type of pull request

[ ] Bug fix (non-breaking change which fixes an issue)
[x] Refactoring (behaviour should not change or only minimally change)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[x] There are changes to the (Java) code
[ ] There are changes to the taclet rule base
[ ] There are changes to the deployment/CI infrastructure (gradle, github, ...)
[ ] Other:

Ensuring quality

[x] I made sure that introduced/changed code is well documented (javadoc and inline comments).
[ ] I made sure that new/changed end-user features are well documented (https://github.com/KeYProject/key-docs).
[ ] I added new test case(s) for new functionality.
[ ] I have tested the feature as follows: ...
[ ] I have checked that runtime performance has not deteriorated.

Additional information and contact(s)

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 37.77%. Comparing base (1fb0c10) to head (7472e4a). Report is 114 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3430 +/- ## ========================================= Coverage 37.77% 37.77% + Complexity 17031 17030 -1 ========================================= Files 2076 2076 Lines 126950 126953 +3 Branches 21381 21381 ========================================= + Hits 47952 47954 +2 Misses 73092 73092 - Partials 5906 5907 +1 ```

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