Refactor and Cleanup

[Sid-Bhatia-0]

1. Make agent color a type parameter of Agent struct.
2. Randomize agent starting direction in SequentialRooms
3. Standarize field-names terminal_reward and terminal_penalty when appropriate, in place of using different names like goal_reward, obstacle_reward, penalty, etc...
4. Rename Pickup action to PickUp
5. Specify the dimension size of 2 as the type parameter for CartesianIndex and CartesianIndices
6. Remove dependency on Colors.jl
7. Clean up imports and exports
8. Miscellaneous cleanup

[codecov-io]

Codecov Report

Merging #99 (93b1d3b) into master (8f67fa5) will increase coverage by 0.49%. The diff coverage is 69.84%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   80.86%   81.36%   +0.49%     
==========================================
  Files          16       16              
  Lines         737      735       -2     
==========================================
+ Hits          596      598       +2     
+ Misses        141      137       -4     

Impacted Files	Coverage Δ
src/GridWorlds.jl	100.00% <ø> (ø)
src/actions.jl	100.00% <ø> (ø)
src/makie_rendering.jl	0.00% <ø> (ø)
src/terminal_rendering.jl	0.00% <0.00%> (ø)
src/objects.jl	40.54% <33.33%> (+3.95%)	:arrow_up:
src/envs/dynamicobstacles.jl	97.50% <75.00%> (ø)
src/abstract_grid_world.jl	72.13% <100.00%> (+8.36%)	:arrow_up:
src/envs/collectgems.jl	100.00% <100.00%> (ø)
src/envs/doorkey.jl	100.00% <100.00%> (ø)
src/envs/emptyroom.jl	100.00% <100.00%> (ø)
... and 4 more

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