[FEATURE] Observation concatenation.

[RuanJohn]

Please describe the purpose of the feature. Is it related to a problem?

When using feedforward policies, partial observability can be handled by using multiple observations instead of only the current one. This is achieved by concatenating the previous n-1 observations to observations to the current observation.

Describe the solution you'd like

An additional environment wrapper that keeps track if the previous n-1 environment observations and is able to concatenate then to be used as the input for a feedforward policy. For example, if n=1, then only the current observation is used as input, and if n=2, then the current and previous observations are concatenated to form the input.

How do we know when implementation of this feature is complete?

Checklist:

• Wrapper is implemented and working
• Tests are written for the wrapper
• The effect on system performance is benchmarked and reported.