Cleanup naming convention

[Sid-Bhatia-0]

Clean up naming convention. Use env to represent an instance of AbstractGridWorld (instead of w or 'gw`) because it represents an environment. This is consistent with the rest of the packages in the ecosystem.

[codecov-io]

Codecov Report

Merging #75 (aad3c67) into master (23f9514) will increase coverage by 0.26%. The diff coverage is 92.64%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   70.81%   71.07%   +0.26%     
==========================================
  Files          14       14              
  Lines         490      491       +1     
==========================================
+ Hits          347      349       +2     
+ Misses        143      142       -1     

Impacted Files	Coverage Δ
src/grid_world_base.jl	72.09% <0.00%> (ø)
src/render_in_terminal.jl	0.00% <0.00%> (ø)
src/abstract_grid_world.jl	87.50% <86.36%> (ø)
src/envs/collectgems.jl	100.00% <100.00%> (ø)
src/envs/doorkey.jl	100.00% <100.00%> (+1.85%)	:arrow_up:
src/envs/dynamicobstacles.jl	100.00% <100.00%> (ø)
src/envs/emptygridworld.jl	100.00% <100.00%> (ø)
src/envs/fourrooms.jl	100.00% <100.00%> (ø)
src/envs/gotodoor.jl	100.00% <100.00%> (ø)

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[Sid-Bhatia-0]

To formulate a consistent naming convention, I also propose the following:

1. Use env to represent environments.
2. Use w as the shorthand for world to consistently represent instances of GridWorldBase (right now some files use both world and w within the same file to represent instances of GridWorldBase (already containing a world field, see point 3. below)
3. Change field-name world of GridWorldBase struct to something like grid. The scope of the word world is a little vague and thus is overused in several different contexts. So GridWorldBase would become something like the following:

   struct GridWorldBase{O} <: AbstractArray{Bool, 3}
   grid::BitArray{3}
   objects::O
   end

   The word grid seems more appropriate than world to represent a multi-dimensional array. (and grid also gels with the Grid part of GridWorlds) Then the name world (or its shorthand w as is already used at several places) can be used to represent an entity that contains:

4. A grid field (a multi-dimensional array representing the layout of the world) and
5. An objects field (a tuple of objects present in the world)