change return type of Property::declare{Read,Update}Handler from void to Property&,

The reason is that this way one can write code like this:
```C++
  declareProperty("trackMaxChi2", m_trackMaxChi2 = 25.)
    ->declareUpdateHandler( [this](Property&){ this->m_trackChi = std::sqrt(this->m_trackMaxChi2); } )
    .useUpdateHandler();
```
this keeps m_trackChi in sync with m_trackMaxChi2. Without the call
to useUpdateHandler, if there is no entry in the JobOptionsSvc,
m_trackChi would not be set, and one would have to duplicate the
code to 'derive' m_trackChi from m_trackMaxChi2 just to initialize
it to the proper default.

Fixes GAUDI-1160.

See merge request !85

GaudiKernel/GaudiKernel/Property.h

@@ -67,17 +67,17 @@ public:
   const std::function<void(Property&)>& updateCallBack() const { return m_updateCallBack; }
 
   /// set new callback for reading
-  virtual void  declareReadHandler   ( std::function<void(Property&)> fun ) ;
+  virtual Property&  declareReadHandler   ( std::function<void(Property&)> fun ) ;
   /// set new callback for update
-  virtual void  declareUpdateHandler ( std::function<void(Property&)> fun ) ;
+  virtual Property&  declareUpdateHandler ( std::function<void(Property&)> fun ) ;
 
   template< class HT >
-  inline void declareReadHandler( void ( HT::* MF ) ( Property& ) , HT* instance )
-  { declareReadHandler( [=](Property& p) { (instance->*MF)(p); } ) ; }
+  inline Property& declareReadHandler( void ( HT::* MF ) ( Property& ) , HT* instance )
+  { return declareReadHandler( [=](Property& p) { (instance->*MF)(p); } ) ; }
 
   template< class HT >
-  inline void declareUpdateHandler( void ( HT::* MF ) ( Property& ) , HT* instance )
-  { declareUpdateHandler ( [=](Property& p) { (instance->*MF)(p); } ); }
+  inline Property& declareUpdateHandler( void ( HT::* MF ) ( Property& ) , HT* instance )
+  { return declareUpdateHandler ( [=](Property& p) { (instance->*MF)(p); } ); }
 
   /// use the call-back function at reading
   virtual void useReadHandler   () const ;

GaudiKernel/src/Lib/Property.cpp

@@ -61,16 +61,18 @@ Property::Property
 // ============================================================================
 // set new callback for reading
 // ============================================================================
-void  Property::declareReadHandler( std::function<void(Property&)> fun )
+Property&  Property::declareReadHandler( std::function<void(Property&)> fun )
 {
   m_readCallBack = std::move(fun);
+  return *this;
 }
 // ============================================================================
 // set new callback for update
 // ============================================================================
-void  Property::declareUpdateHandler ( std::function<void(Property&)> fun )
+Property&  Property::declareUpdateHandler ( std::function<void(Property&)> fun )
 {
   m_updateCallBack = std::move(fun);
+  return *this;
 }
 // ============================================================================
 // use the call-back function at reading