Add conversion rules for Float/Double negation

[WolframPfeifer]

It seems that there are two rules missing which convert from javaUnaryMinusFloat/Double to negFloat/negDouble. Without these rules, KeY (and also the SMT float conversion) can not prove very much when a negation occurs, since the function will just be left uninterpreted.

This PR just adds the two missing rules.

Caveat: Somebody with more knowledge about floating points should double-check this PR (I think it should be sound but am not 100% sure).

Type of pull request

-[x] 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
  -[x] 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.

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

[WolframPfeifer]

@JonasKlamroth @samysweb I just created this PR for our small fix.