unaryPlus overload for group elements

[Iaroslav Postovalov]

Is your feature request related to a problem? Please describe.

It would be nice to have explicit unary plus prefix op for group elements, etc.

In my usecase, it would be nice for code consistency (even assuming that this operator will just return this)

https://github.com/mipt-npm/kmath/blob/2723c376d9b123cb0d311411d6757d6fb8f379b7/kmath-kotlingrad/src/main/kotlin/kscience/kmath/kotlingrad/ScalarsAdapters.kt#L88

Describe the solution you'd like
Add unaryPlus for SFun.

[Iaroslav Postovalov]

Thanks