Resolve "Emplace API inconsistent with ProcessorTrackerFeature/Landmark functions"

Closes #191 (closed)

src/factor/factor_base.cpp

@@ -99,21 +99,25 @@ void FactorBase::remove()
 
 const Eigen::VectorXs& FactorBase::getMeasurement() const
 {
+    assert(getFeature() != nullptr && "calling getMeasurement before linking with a feature");
     return getFeature()->getMeasurement();
 }
 
 const Eigen::MatrixXs& FactorBase::getMeasurementCovariance() const
 {
+    assert(getFeature() != nullptr && "calling getMeasurementCovariance before linking with a feature");
     return getFeature()->getMeasurementCovariance();
 }
 
 const Eigen::MatrixXs& FactorBase::getMeasurementSquareRootInformationUpper() const
 {
+    assert(getFeature() != nullptr && "calling getMeasurementSquareRootInformationUpper before linking with a feature");
     return getFeature()->getMeasurementSquareRootInformationUpper();
 }
 
 CaptureBasePtr FactorBase::getCapture() const
 {
+    assert(getFeature() != nullptr && "calling getCapture before linking with a feature");
     return getFeature()->getCapture();
 }