Replaced helper function sign().

[lewie-donckers]

Replaced helper function sign().

I saw three (minor) problems with it:

• it has a bit of an odd (and undocumented) return type/value.
• it is basically a inferior version of std::signbit().
• it was only used in one location to check if two values have the same sign.

So I replaced it with a function that does exactly that with a descriptive name and an intuitive return type/value.

[lewie-donckers]

I wouldn't just remove the old sign function. There are a lot of places where it could be used.

If you grep for sign you see a lot of these constructions

copysign(1.0, somevalue)

Which could be replaced by

sign(somevalue)

While I think refactoring those copysign() calls is a great idea, I don't think we should keep the function because we might need it in the future.

We know it existed (we can refer to it in an issue to request the refactor) so we can retrieve it when we want to use it again. But for now, it is dead code.