ICoordConverter: reintroduce constness

With 8ececffa the method of IEnvironment

  [[nodiscard]] virtual const ICoordConverter* GetConverter() const = 0;

became

  [[nodiscard]] virtual ICoordConverter* GetConverter() = 0;

As discussed in the sync-meeting on the 1/17/25, this change was necessary due to a caching at the implementation side. The group came to the conclusion that we should not make our interfaces reflect implementation details. Hence, the interface should become again a const method and instead the implementation has to take care of that (e.g. using mutables inside).