foldSafe() for negative values

[PavelBal]

          Theoretically, we could add an optional boolean argument telling if the compared values are supposed to be negative.

If so, we would replace all values which are >-epsilon with -epsilon.
But I am not aware of any practical use case for this :)

Originally posted by @Yuri05 in #130 (comment)

[PavelBal]

Not prio, only if we have a use-case.