Raise DeprecationWarning at Environment class

Environment, which was used by the old (< v8.0) code base and still used by SelectiveReplayBuffer, is no more maintained and already almost deprecated.

Until SelectiveReplayBuffer, where episodes can be deleted selectively, is re-implemented on the new (>= v8.0) code base, DeprecationWarning should be raised.

Also it should be described at its docstring.