Python State functions composition

[AntoinePrv]

State function could be composed easily to create new ones.

• Sum, product, power (also with scalars) of rewards functions
• Logical operators of early termination functions
• Cumulative Reward
• TupleObservationFunction and DictObservationFunction for observation functions
• Make Environment automatically create Dict/Tuple obs func if given a tuple/dict.

[gasse]

I changed the milestone to 0.2.0, since we'll need to be able to query several observations at once in order to reproduce learn2branch

[AntoinePrv]

Implemented the Dcit and Tuple observation function for the Python API in d456802 currently in v0.2.x.
For the rest, I suggest moving to v0.3.