refs #314, fromQVariant / location of toQVariant

* renamed fromQVariant to convertFromQVariant, the "from" methods imply the object is changed (fromJson, fromString ..)
* implemented fromQVariant - changing the object by QVariant
* moved toQVariant into PQ template
* free function setFromQVariant

src/blackmisc/pqfrequency.h

@@ -35,12 +35,6 @@ namespace BlackMisc
             //! \copydoc CPhysicalQuantity(const QString &unitString)
             CFrequency(const QString &unitString) : CPhysicalQuantity(unitString) {}
 
-            //! \copydoc CValueObject::toQVariant
-            virtual QVariant toQVariant() const override
-            {
-                return QVariant::fromValue(*this);
-            }
-
             //! Virtual destructor
             virtual ~CFrequency() {}
         };