Constructor for PQ by string (e.g. 100km/h),

motivated by MS review https://dev.vatsim-germany.org/issues/212#note-6
refs #212

src/blackmisc/pqfrequency.h

@@ -23,6 +23,8 @@ namespace BlackMisc
             //! Init by double value
             CFrequency(double value, const CFrequencyUnit &unit) : CPhysicalQuantity(value, unit) {}
 
+            //! \copydoc CPhysicalQuantity(const QString &unitString)
+            CFrequency(const QString &unitString) : CPhysicalQuantity(unitString) {}
 
             //! \copydoc CValueObject::toQVariant
             virtual QVariant toQVariant() const override