Fix abstract_learner for multiplayer games

[jeremiahpslewis]

This pull request fixes the abstract_learner for multiplayer games. It adds a new function forward(learner::L, env::E, player::Symbol) that takes a learner, environment, and player symbol as arguments and sends the state to RLCore.forward(Learner, State). This ensures that the learner can handle multiplayer games correctly.

It also cleans up a few other minor issues along the way.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 59.32%. Comparing base (93a13d3) to head (d6d9813).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1054      +/-   ##
==========================================
- Coverage   59.34%   59.32%   -0.03%     
==========================================
  Files          73       73              
  Lines        2789     2795       +6     
==========================================
+ Hits         1655     1658       +3     
- Misses       1134     1137       +3     

Files	Coverage Δ
...nforcementLearningCore/src/core/stop_conditions.jl	80.64% <100.00%> (ø)
...ntLearningCore/src/policies/learners/td_learner.jl	91.30% <ø> (ø)
...ningCore/src/policies/learners/abstract_learner.jl	61.53% <50.00%> (+1.53%)	⬆️
...rcementLearningCore/src/policies/q_based_policy.jl	60.00% <50.00%> (-11.43%)	⬇️