code: Optional<Boolean> getBoolean(String key) {
          Object value = get(key);
  
          if (value == null) {
              return Optional.empty();
          }
  
          if (value instanceof Boolean) {
              return Optional.of((Boolean) value);
          }
          else if (value instanceof String) {
              return Optional.of(Boolean.parseBoolean((String)value));
          }
          else {
              throw new InvalidOntopConfigurationException("A boolean was expected: " + value);
          }
      }
docs: Returns the boolean value of the given key.