Collect "magic strings" #91
Description
Through particularly the RL code, several strings are used to ensure the communication between samplefactory and our code works well.
This includes some strings that we used to eg define the "rnn_state" as an input in our configs, and on the other hand strings that samplefactory assumes are present in eg result dictionaries.
While some of them could potentially be removed by a refactoring completely, I think a more safe way to work with them would be to have a python file where all of them are defined as constants, and then referenced in the code instead of being typed.