Generate easier POs for non-trivial diverges clauses

[Drodt]

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

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

Hence, we have a proper cut.

Type of pull request

• Bug fix (non-breaking change which fixes an issue)
• 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)
• 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

• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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[FliegendeWurst]

Shouldn't this be a copy of clauses.requires? Previously that map was used when constructing the FunctionalOperationContract.