Add decision_lexico_order parameter to agent_objectives table

[tsmbland]

Description

Adding the decision_lexico_order parameter to the objectives table, and some validation

So far the validation is checking:

• if decision rule is lexico, the decision_lexico_order must be specified, otherwise it mustn't be
• the number of objectives will depend on the decision rule: if "single" there should be one objective in each year, if "weighted" there should be more than 1, and if "lexico" there should be 2 and these should have weights 1 and 2 (i.e. a primary and a secondary)

There is still more validation to do for the objectives such as ensuring weights add to 1 (see #458), so I imagine a lot of this will change when I come to tackle that

Fixes #457

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

All modified and coverable lines are covered by tests ✅

Project coverage is 95.47%. Comparing base (78b707c) to head (3200df0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
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+ Coverage   95.40%   95.47%   +0.06%     
==========================================
  Files          31       31              
  Lines        4588     4658      +70     
  Branches     4588     4658      +70     
==========================================
+ Hits         4377     4447      +70     
  Misses        106      106              
  Partials      105      105              

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[tsmbland]

I wonder if there might be a better way to structure the data, such as storing the objectives within DecisionRule? Not sure

[alexdewar]

LGTM!

I guess you could store the objectives inside the decision rule enum, but I'd worry that that might make the code to access them a bit convoluted. The extra safety would be nice though. Maybe let's see how we end up using them and see if it makes sense further down the line.