Removing constness causes problems with some no-default-constructor classes. Add dummy default constructors or expose them to public for Cython wrapper only.

Maybe a Cython bug? Both object and pointer object appeared next to each other in the generated cpp file, e.g.
   JointMarginal p0;
   JointMarginal* p1;
With correct constness, only the pointer object shows up.

Maybe related: https://groups.google.com/forum/#!topic/cython-users/HB5yxgKQ6wc

gtsam/navigation/AHRSFactor.h

@@ -38,13 +38,13 @@ class GTSAM_EXPORT PreintegratedAhrsMeasurements : public PreintegratedRotation
   Vector3 biasHat_; ///< Angular rate bias values used during preintegration.
   Matrix3 preintMeasCov_; ///< Covariance matrix of the preintegrated measurements (first-order propagation from *measurementCovariance*)
 
-  /// Default constructor, only for serialization
-  PreintegratedAhrsMeasurements() {}
-
   friend class AHRSFactor;
 
  public:
 
+  /// Default constructor, only for serialization and Cython wrapper
+  PreintegratedAhrsMeasurements() {}
+
   /**
    *  Default constructor, initialize with no measurements
    *  @param bias Current estimate of acceleration and rotation rate biases

gtsam/navigation/CombinedImuFactor.h

@@ -115,7 +115,6 @@ public:
    */
   Eigen::Matrix<double, 15, 15> preintMeasCov_;
 
-  PreintegratedCombinedMeasurements() {}
 
   friend class CombinedImuFactor;
 
@@ -123,6 +122,9 @@ public:
   /// @name Constructors
   /// @{
 
+  /// Default constructor only for serialization and Cython wrapper
+  PreintegratedCombinedMeasurements() {}
+
   /**
    *  Default constructor, initializes the class with no measurements
    *  @param bias Current estimate of acceleration and rotation rate biases

gtsam/navigation/ImuFactor.h

@@ -78,13 +78,13 @@ protected:
   Matrix9 preintMeasCov_; ///< COVARIANCE OF: [PreintPOSITION PreintVELOCITY PreintROTATION]
   ///< (first-order propagation from *measurementCovariance*).
 
-  /// Default constructor for serialization
+public:
+
+  /// Default constructor for serialization and Cython wrapper
   PreintegratedImuMeasurements() {
     preintMeasCov_.setZero();
   }
 
-public:
-
  /**
    *  Constructor, initializes the class with no measurements
    *  @param bias Current estimate of acceleration and rotation rate biases

gtsam/nonlinear/Marginals.h

@@ -48,6 +48,10 @@ protected:
 
 public:
 
+  /// Default constructor only for Cython wrapper
+  Marginals(){}
+
+  /// Copy constructor
   Marginals(const Marginals&) = default;
 
   /** Construct a marginals class.
@@ -93,6 +97,10 @@ protected:
   FastMap<Key, size_t> indices_;
 
 public:
+  /// Default constructor only for Cython wrapper
+  JointMarginal() {}
+
+  /// Copy constructor
   JointMarginal(const JointMarginal&) = default;
 
   /** Access a block, corresponding to a pair of variables, of the joint