code: public OSchemaHelper oProperty(String propertyName, OType type)
  	{
  		checkOClass();
  		lastProperty = lastClass.getProperty(propertyName);
  		if(lastProperty==null)
  		{
  			lastProperty = lastClass.createProperty(propertyName, type);
  		}
  		else
  		{
  			if(!type.equals(lastProperty.getType()))
  			{
  				lastProperty.setType(type);
  			}
  		}
  		return this;
  	}
docs: Create {@link OProperty} if required
  @param propertyName property name to create
  @param type type of property to create
  @return this helper