Fix ExpectData compiler warnings (gazebosim#335)

Use default assignment operator.
Addresses gazebosim#332.

Signed-off-by: Michael Carroll <michael@openrobotics.org>

include/ignition/physics/SpecifyData.hh

@@ -57,6 +57,9 @@ namespace ignition
  /// the newly copied MapData.
  public: ExpectData(const ExpectData &_other);
 
+ /// \brief Copy assignment operator.
+ public: ExpectData& operator=(const ExpectData &_other) = default;
+
  /// TODO(anyone) Implement move constructor and assignment operator. Due
  /// to the multiple inheritence used to implement SpcifyData, care must be
  /// taken when implementing move constructs to avoid calling a move