Make `eval` function private, or rename it?

[albertz]

Similar argument as #63. I think there should be nothing why this would ever be needed.

Although I can see that we have some legacy code (e.g. SpecAugment) which we might want to wrap initially, where we would need it.

And also, whenever we call external code, sth like this would be needed. Although for someone unfamiliar with RETURNN, eval is maybe confusing for this, and sth like wrap_external_function or so makes more sense?

[albertz]

This is done now.