Improve use of enum in IncomeComparisonOperator

[itsme-zeix]

If have time can consider making these greater/equal/less than behavior more type-safe (and more OOP) by implementing enums! Maybe something like this:

public enum IncomeComparisonOperator {
    EQUAL("="),
    GREATER_THAN(">"),
    LESS_THAN("<");
    // some assign symbol logic
}

Originally posted by @iamdiluxedbutcooler in #95 (comment)