Revise which minimization results to accept for the Hesse error refinement algorithm

[AdrianDBS]

Currently, if an IMinimizationResult doesn't carry a FunctionMinimum -- which means it's certainly not a valid result -- we are still accepting it for the Hesse method but (silently) return the input result. We should handle this in a cleaner way. Options are:

• We could simply allow to execute Hesse on any result. This would require extending the algorithm to really accept "invalid results".
• We could restrict the method to accept only valid results (via a new IValidMinimizationResult interface or the like)