RNG is not Singleton

[Peter]

We allow construction through the compiler generated copy constructor.

This is extra severe since the destructor destroys the instance, which is illustrated by this code snippet

RNG* rng=RNG::instance();
rng->seed_from_devurandom();

RNG* rng2 = new RNG(*rng);
std::cout << rng2->name() << std::endl;
delete rng2;

std::cout << rng->name() << std::endl;

[Jari Häkkinen]

There is also a flaw in that the destructor is public! Anyone can delete the content of the supposedly singleton RNG, but the instance_ is not reset to NULL. This would allow creation of new RNG singletons, but all active RNG*'s would become invalid.

All constructors should be private, and the destructor must also be private.

[Jari Häkkinen]

(In [852]) Fixes #228.