Refactor target to have ground truth pointer

[dyc3]

• add agent test
• refactor target to contain a pointer to the ground truth pokemon instead of a copy

This PR is an effort to consolidate how pokemon are accessed into one easy mechanism. An added bonus is a potentially reduced lifetime for values copied when creating battle contexts for Agents, which would mean less memory usage over general usage.

Now the code more clearly defines where pokemon are copied, and where pokemon are passed by reference.

Another bonus, it closes #294

[codecov[bot]]

Codecov Report

Merging #295 (3f14944) into main (fcad799) will increase coverage by 0.4623%. The diff coverage is 98.4375%.

:exclamation: Current head 3f14944 differs from pull request most recent head e0d1487. Consider uploading reports for the commit e0d1487 to get more accurate results

@@               Coverage Diff                @@
##               main       #295        +/-   ##
================================================
+ Coverage   95.1690%   95.6314%   +0.4623%     
================================================
  Files            14         14                
  Lines          1449       1465        +16     
================================================
+ Hits           1379       1401        +22     
+ Misses           60         53         -7     
- Partials         10         11         +1     

Impacted Files	Coverage Δ
battle.go	91.3157% <98.2758%> (+0.4067%)	:arrow_up:
transactions.go	91.3793% <100.0000%> (-0.0493%)	:arrow_down:
pokemon.go	97.2602% <0.0000%> (+4.1095%)	:arrow_up:

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