Add helper functions for multi-objective optimization + refactoring (#6473)

Add helper functions for multi-objective optimization + refactoring (#6473)

add 3 helper functions
judge whether a node is optimizable
make strong improvement expression according to optimization objective
make weak improvement expression according to optimization objective
optChecker is now created by optimizationSolver instead of the minimize/maximize functions
Slightly refactors function signatures so that they are accepting OptimizationObjective instead of accepting target, type in separate parameters.