Improve consistency between mixes in multi_mix

[BDonnot]

Is your feature request related to a problem? Please describe.

Some function with the MultimixEnv have an unexpected behavior and only affect some mixes in the environment (instead of being properly forwarded to all mixes)

This could be avoided at the creation of the first environment if some things, such as:

• the reward helper
• the action_space
• the observation_space
• Something else?
  Would be reused for all mixes.

Describe the solution you'd like

A clear and concise description of what you want to happen.
The solution would be, when à multi mix is created, to reuse the attributes from the first environnement in the other mix.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

An alternative is to loop through every mix to initialize things, like this for example:

for mix in multi_mix:
   cr = mix.get_reward_instance()
   cr.addReward(...)
  ...

[BDonnot]

More generally, it would even be better to have the "MultiMix" as a type of "Chronics" and NOT as a type of environment, and thus get rid of the all the "MultiMixEnv" specificities and overloading.