Update to use saref:PropertyValue

SAREF now includes the saref:PropertyValue class to represent property values.

PROPOSAL:

• To replace s4grid:PropertyValue with saref:PropertyValue.
• To replace s4grid:hasValue with saref:hasValue.
• To replace s4grid:hasUnit with saref:isMeasuredIn.
• To replace s4grid:hasPropertyValue with saref:hasPropertyValue.
• To replace s4grid:hasProperty with saref:isValueOfProperty.

Changes have to be performed in the OWL code, TS, diagrams, documentation and examples.