Move `decision_lexico_tolerance` parameter to `agents` table

[tsmbland]

See #442 for details of the new proposed schema for agent parameters

This parameter should be defined at the Agent level rather than the objective level.

We should put this inside the DecisionRule enum, which is already saved under the decision_rule attribute of Agent

Validation:

• If the decision rule is "lexico", this should be specified
• If the decision rule is anything else, it shouldn't be specified
• Presumably this should be positive