Add doxygen for GMM

gtsam/hybrid/GaussianMixture.h

@@ -24,8 +24,7 @@
 #include <gtsam/linear/GaussianConditional.h>
 
 namespace gtsam {
-class GaussianMixture
-    : public HybridFactor,
+class GaussianMixture : public HybridFactor,
                         public Conditional<HybridFactor, GaussianMixture> {
  public:
   using This = GaussianMixture;
@@ -38,6 +37,14 @@ class GaussianMixture
   Conditionals conditionals_;
 
  public:
+  /**
+   * @brief Construct a new Gaussian Mixture object
+   *
+   * @param continuousFrontals the continuous frontals.
+   * @param continuousParents the continuous parents.
+   * @param discreteParents the discrete parents. Will be placed last.
+   * @param conditionals a decision tree of GaussianConditionals.
+   */
   GaussianMixture(const KeyVector &continuousFrontals,
                   const KeyVector &continuousParents,
                   const DiscreteKeys &discreteParents,