Move `creation_callback` arg to be final arg of `generate` function

[elliotcmorris]

Spawned from this conversation.

Move the creation_callback argument in the generate method to the end of the function, so users can opt to omit it more often.

The reasoning is that it's rarely used, and it's a more complicated type than your bog standard ints and bools, so isn't very intuitive.

Default logging to std::err if non supplied.