Add script for MCEIRL Algorithm

[AdamGleave]

The maximum causal entropy IRL algorithm is implemented in https://github.com/HumanCompatibleAI/imitation/blob/master/src/imitation/algorithms/mce_irl.py but there is not currently a training script for it in https://github.com/HumanCompatibleAI/imitation/tree/master/src/imitation/scripts