undocumented assert about uniqueness of user input

[tilsche]

The registry seems to assert that created elements get unique keys from the user.

https://github.com/tud-zih-energy/otf2xx/blob/master/include/otf2xx/registry.hpp#L286

Given that this can be triggered from usage, and it doesn't seem critical to check here, it should rather be an if/throw.

It would also be nice to have this precondition properly documented.