implement Multi-Objective / Goal system

[fabioseel]

This is probably the biggest TODO.
I think there won't be a way around modifying the library to our needs, I am unsure whether weird side-effects could arise. Might be worth implementing some safety checks / tests first and approach this test-driven.

Definitely will affect the Learner class / backward step.

[fabioseel]

• Check whether implementing an own Learner would be sufficient
  • potentially could allow to use completely custom model
    • yes and no. Actor critic is used in several other places, but its not completely outrageous...
    • more importantly: the RNN handling might be sth we want, so it might be worth keeping it
  • potentially could allow better initialization than write etc
    • seems hard without changing major parts of sample-factory
• if so, find way to inject it or if we have to use our own cloned repo
  • with our own repo, we'd at least only have to maintain the Learner and can just rebase on the 'true' repo every now and then