feature: non-context-manager-based access to the Golem engine

[shadeofblue]

to allow more freedom in the usage of the Golem engine, there should be an easy way to instantiate Golem and to be able to explicitly shut it down, without having to use the context manager syntax.

additional thoughts:

• maybe there could be an option to automatically close Golem when e.g. nothing else is done with it? (e.g. on no explicit close by the time the program exists or when Golem instance leaves the scope) -> e.g. __del__ implementation or an atexit handler