Implement an integrity check on configuration time

RulesExpression plugins should have a method integrityCheck() that determines if the current configuration setup is sane and no dependencies are missing or configured data types do not match.

Proposed solution: add a "configuration state" class that holds all data definitions of variables that are used in a rule. Add ->integrityCheck() methods to expression objects that throw exception in case an expression is misconfigured. The configuration state is used during traversing nested expression objects to determine which variable are available at which level in the expression tree.