C++ State function composition

[AntoinePrv]

Describe the problem or improvement suggested

Implement the same Python API #49 in C++

Describe the solution you'd like

Reward function all derive from the same ABC, so this could be done with virtual calls (rather than templates) and bound to Python (minimize code duplication).

For observation functions, template (and hence code duplication) seem necessary.

Describe alternatives you've considered