BUG: Honor StopOptimization from IterationEvent in v4 optimizers

Add an immediate m_Stop check after InvokeEvent(IterationEvent()) in
ExhaustiveOptimizerv4, PowellOptimizerv4, and
OnePlusOneEvolutionaryOptimizerv4. Previously these optimizers would
continue advancing after an observer called StopOptimization() (or
StopWalking()) during IterationEvent, making it impossible to halt
from a callback.

This is the companion change to cf929f2 which added the same
pattern to the GradientDescentOptimizerv4 family. All v4 optimizers
now consistently stop before taking another step when Stop is
requested from an IterationEvent observer.

Author: blowekamp

Modules/Numerics/Optimizersv4/include/itkExhaustiveOptimizerv4.hxx

@@ -134,6 +134,11 @@ ExhaustiveOptimizerv4<TInternalComputationValueType>::ResumeWalking()
                                << "@ index " << this->GetCurrentIndex() << " value is " << m_CurrentValue;
 
     this->InvokeEvent(IterationEvent());
+    if (m_Stop)
+    {
+      this->StopWalking();
+      break;
+    }
     this->AdvanceOneStep();
     this->m_CurrentIteration++;
   }

Modules/Numerics/Optimizersv4/include/itkOnePlusOneEvolutionaryOptimizerv4.hxx

@@ -257,6 +257,13 @@ OnePlusOneEvolutionaryOptimizerv4<TInternalComputationValueType>::StartOptimizat
     }
 
     this->InvokeEvent(IterationEvent());
+    if (m_Stop)
+    {
+      m_StopConditionDescription.str("");
+      m_StopConditionDescription << this->GetNameOfClass() << ": "
+                                 << "StopOptimization() called";
+      break;
+    }
     itkDebugMacro("Current position: " << this->GetCurrentPosition());
   }
   if (this->m_CurrentIteration >= m_MaximumIteration)

Modules/Numerics/Optimizersv4/include/itkPowellOptimizerv4.hxx

@@ -498,6 +498,12 @@ PowellOptimizerv4<TInternalComputationValueType>::StartOptimization(bool /* doOn
     }
 
     this->InvokeEvent(IterationEvent());
+    if (m_Stop)
+    {
+      m_StopConditionDescription << "StopOptimization() called";
+      this->InvokeEvent(EndEvent());
+      return;
+    }
   }
 
   m_StopConditionDescription << "Maximum number of iterations exceeded. "