Add year field to objectives

[tsmbland]

Description

This is a nice easy, self-contained change to kick off the plans described in #442

All I'm doing here is adding a year field to the existing agent_objectives table. The actual work of adding the field is tiny, I just added a year attribute to the AgentObjective struct, a year column to the csv, and I believe the process of reading in the new data is all automatic.

Most of the work here is to do with validation:

• checking that the year is a valid milestone year
• checking that all agents have at least one objective for each year

I've copied each objective in the table to apply to both 2020 and 2030, but long run we want a better approach (see #444)

Fixes # (issue)

Type of change

• Bug fix (non-breaking change to fix an issue)
• 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

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

Further checks

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

[codecov]

Codecov Report

Attention: Patch coverage is 93.61702% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.50%. Comparing base (3ff209b) to head (adf2710).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/input.rs	85.71%	0 Missing and 1 partial ⚠️
src/input/agent.rs	50.00%	0 Missing and 1 partial ⚠️
src/input/agent/objective.rs	97.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #446      +/-   ##
==========================================
+ Coverage   95.49%   95.50%   +0.01%     
==========================================
  Files          31       31              
  Lines        4479     4515      +36     
  Branches     4479     4515      +36     
==========================================
+ Hits         4277     4312      +35     
- Misses         99      100       +1     
  Partials      103      103              

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[alexdewar]

LGTM. I've made a couple of small suggestions.