message: Add set getter methods in DynamicVariableRelation to correspond with StaticRelation
diff: #include "axom/config.hpp"
  #include "axom/slic.hpp"
  +#include "axom/slam/policies/PolicyTraits.hpp"
  +
  #include "axom/slam/Set.hpp"
  #include "axom/slam/Relation.hpp"
  @@ -104,6 +106,20 @@ public:
  return sz;
  }
  + bool hasFromSet() const
  + {
  + return !policies::EmptySetTraits<SetType>::isEmpty(m_fromSet);
  + }
  + SetType* fromSet() { return m_fromSet; }
  + const SetType* fromSet() const { return m_fromSet; }
  +
  + bool hasToSet() const
  + {
  + return !policies::EmptySetTraits<SetType>::isEmpty(m_toSet);
  + }
  + SetType* toSet() { return m_toSet; }
  + const SetType* toSet() const { return m_toSet; }
  +
  SetPosition fromSetSize() const { return m_relationsVec.size(); }
  bool isValid(bool verboseOutput = false) const;
  