POMDPTools, Graphs, and CI Updates

[dylan-asmar]

• Updated to use POMDPTools
• Changed LightGraphs to Graphs
• Updated gitignore and cleaned up files
• Changed min supported Julia version to 1.6
• Updated CI and docs workflows
• Updated badges on readme and added citation block
• Version bump

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (26f7aa6) 93.60% compared to head (0c75517) 93.76%.

Files	Patch %	Lines
src/fvmcts/fv_mcts_vanilla.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   93.60%   93.76%   +0.16%     
==========================================
  Files           5        5              
  Lines         375      417      +42     
==========================================
+ Hits          351      391      +40     
- Misses         24       26       +2     

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