Use absolute value for balancetype using targetP

[obrix]

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• [X ] The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Following discussion with @annetill we should make sure factor is an absolute value when using proportional_to_generation_p balance type. This did not make it in the original PR.

What is the current behavior? (You can also link to an open issue here)

No absolute value factor for proportional_to_generation_p balancetype.

What is the new behavior (if this is a feature change)?

Factor value is now absolute for proportional_to_generation_p balancetype.

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

• The Breaking Change or Deprecated label has been added
• The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)