Add StochasticEnv wrapper

[Sid-Bhatia-0]

This wrapper allows us to convert a minimally written deterministic environment like EmptyGridWorld, and add customized stochastic behaviour without having to create a new environment that is mostly similar and only slightly different.

For example, this wrapper can be used to easily add randomized reset behaviour (random agent start position and direction at each call to reset!), or make the agent take the intended action only probabilistically (as if it is a drunken agent).

[Sid-Bhatia-0]

@findmyway , I think it will be helpful to add examples like the drunken agent or randomly starting agent (position/direction). Should I add them in tests?

[codecov-io]

Codecov Report

Merging #63 (cfd5cc6) into master (7d32edc) will decrease coverage by 0.50%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   65.23%   64.73%   -0.51%     
==========================================
  Files          14       15       +1     
  Lines         443      448       +5     
==========================================
+ Hits          289      290       +1     
- Misses        154      158       +4     

Impacted Files	Coverage Δ
src/GridWorlds.jl	100.00% <ø> (ø)
src/wrappers.jl	0.00% <0.00%> (ø)
src/envs/doorkey.jl	100.00% <0.00%> (+2.77%)	:arrow_up:

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