Add converters

[findmyway]

• Add a converter discrete2standard_discrete
• Bugfix with EXPLICIT_STOCHASTIC envs. Add a chance player into the players
• Move MultiThreadEnv into RLZoo to avoid loading RLEnvs there.
• Add StochasticEnv to convert environments of EXPLICIT_STOCHASTIC to STOCHASTIC

[codecov-io]

Codecov Report

Merging #109 (d8b0ab1) into master (812c42b) will increase coverage by 0.02%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   70.72%   70.75%   +0.02%     
==========================================
  Files          30       31       +1     
  Lines        1069     1053      -16     
==========================================
- Hits          756      745      -11     
+ Misses        313      308       -5     

Impacted Files	Coverage Δ
src/converters.jl	0.00% <0.00%> (ø)
src/environments/wrappers/wrappers.jl	100.00% <ø> (ø)
src/environments/3rd_party/open_spiel.jl	27.77% <16.66%> (-1.64%)	⬇️
src/environments/wrappers/StochasticEnv.jl	78.26% <78.26%> (ø)
src/environments/examples/KuhnPokerEnv.jl	90.38% <100.00%> (+14.38%)	⬆️
src/base.jl	20.54% <0.00%> (-1.37%)	⬇️
src/environments/3rd_party/atari.jl	64.47% <0.00%> (-1.32%)	⬇️

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