Changed solve to evaluate for implicit function unit tests
Refactored symbolic evaluation in FX base class
Moved EvaluationMX::evaluateMX implementation to FXInternal, removed code duplication
Moved sparsity pattern propagation via known Jacobian sparsity to FXInternal
Moved remaining functions in EvaluationMX to FXInternal to allow overloading
Renamed EvaluationMX -> CallFX