Read in agents-related files

[alexdewar]

Description

This PR adds support for reading in the various agent-related CSV files, viz.: agents.csv, agent_regions.csv and agent_objectives.csv.

Closes #76. Closes #107. Closes #108.

Type of change

• [ ] Bug fix (non-breaking change to fix an issue)
• [x] New feature (non-breaking change to add functionality)
• [ ] Refactoring (non-breaking, non-functional change to improve maintainability)
• [ ] Optimization (non-breaking change to speed up the code)
• [ ] Breaking change (whatever its nature)
• [ ] Documentation (improve or add documentation)

Key checklist

• [x] All tests pass: $ cargo test
• [ ] The documentation builds and looks OK: $ cargo doc

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests added that prove fix is effective or that feature works

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.44262% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.20%. Comparing base (155bc25) to head (fa25e6b). Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/agent.rs	92.85%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #151 +/- ## ========================================== + Coverage 83.45% 85.20% +1.75% ========================================== Files 10 11 +1 Lines 278 338 +60 ========================================== + Hits 232 288 +56 - Misses 46 50 +4 ```

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[alexdewar]

Oops, sorry @Sahil590. I meant to give you a chance to review this. Comments still welcome!