clean up & fix more miscellaneous things

[Sid-Bhatia-0]

1. Remove all assets.
2. Clean up imports in GridWorlds.jl file. No need to import Random & DataStructures.
3. Fix method signatures of act!, get_pretty_tile_map, and get_pretty_sub_tile_map in abstract_grid_world.jl. Rearrange some things in abstract_grid_world.jl.
4. Update sample_empty_position method and replace the one in Snake.
5. Use sample_empty_position instead of sample_two_positions_without_replacement in SingleRoomUndirected.

[codecov-commenter]

Codecov Report

Merging #177 (0d14b7e) into master (b4287b2) will decrease coverage by 0.17%. The diff coverage is 30.43%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
- Coverage   62.25%   62.08%   -0.18%     
==========================================
  Files          27       27              
  Lines        2612     2624      +12     
==========================================
+ Hits         1626     1629       +3     
- Misses        986      995       +9     

Impacted Files	Coverage Δ
src/abstract_grid_world.jl	13.93% <20.00%> (+2.00%)	:arrow_up:
src/envs/single_room_undirected.jl	65.21% <100.00%> (-0.38%)	:arrow_down:
src/envs/snake.jl	72.35% <100.00%> (ø)

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