merge DetectorContext 2/n

Sim/Simulation/ISimulation2D.cpp

@@ -51,6 +51,7 @@ std::vector<const INode*> ISimulation2D::nodeChildren() const
 
 void ISimulation2D::prepareSimulation()
 {
+    m_active_indices = m_detector->active_indices();
     m_detector_context = std::make_unique<DetectorContext>(m_detector.get());
 }
 
@@ -71,9 +72,7 @@ void ISimulation2D::setRegionOfInterest(double xlow, double ylow, double xup, do
 
 size_t ISimulation2D::numberOfElements() const
 {
-    if (!m_detector_context)
-        throw std::runtime_error("Error in numberOfElements(): no detector context");
-    return m_detector_context->numberOfElements();
+    return m_active_indices.size();
 }
 
 std::unique_ptr<IComputation> ISimulation2D::createComputation(const ReSample& re_sample,