code: def isValid(self):
          
          today = DateTime()
          expiry_date = self.getExpiryDate()
          if expiry_date and today > expiry_date:
              return False
          # TODO: Do We really need ExpiryDate + DateExpired? Any difference?
          date_expired = self.getDateExpired()
          if date_expired and today > date_expired:
              return False
  
          date_disposed = self.getDateDisposed()
          if date_disposed and today > date_disposed:
              return False
  
          return True
docs: Returns if the current Reference Sample is valid. This is, the sample
          hasn't neither been expired nor disposed.