Trajectory redesign

[findmyway]

Now trajectory is fully composable. This will make my life much easier to implement JuliaReinforcementLearning/ReinforcementLearningZoo.jl#60

[codecov-commenter]

Codecov Report

Merging #106 into master will increase coverage by 2.66%.
The diff coverage is 83.05%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   42.09%   44.76%   +2.66%     
==========================================
  Files          41       33       -8     
  Lines         803      735      -68     
==========================================
- Hits          338      329       -9     
+ Misses        465      406      -59     

Impacted Files	Coverage Δ
src/components/agents/dyna_agent.jl	0.00% <ø> (ø)
src/utils/circular_array_buffer.jl	87.80% <ø> (-2.44%)	⬇️
src/components/trajectories/trajectory.jl	80.85% <80.43%> (-2.49%)	⬇️
src/components/agents/agent.jl	72.09% <88.88%> (+29.47%)	⬆️
src/components/trajectories/abstract_trajectory.jl	100.00% <100.00%> (+35.29%)	⬆️
src/utils/sum_tree.jl	49.15% <100.00%> (+8.47%)	⬆️
src/extensions/Flux.jl	7.69% <0.00%> (+7.69%)	⬆️

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