Add min/max, delta and average cell voltage sensors

[syssi]

ESP_LOGV(TAG, "Number of cells: %d", cells);
  //   9      0x0C 0xD7      Cell voltage 1                   3287 * 0.001f = 3.287         V
  //   11     0x0C 0xE9      Cell voltage 2                   3305 * 0.001f = 3.305         V
  //   ...    ...            ...
  //   39     0x0C 0xD8      Cell voltage 16                                                V
  float min_cell_voltage = 100.0f;
  float max_cell_voltage = -100.0f;
  float average_cell_voltage = 0.0f;
  uint8_t min_voltage_cell = 0;
  uint8_t max_voltage_cell = 0;
  for (uint8_t i = 0; i < std::min((uint8_t) 16, cells); i++) {
    float cell_voltage = (float) seplos_get_16bit(9 + (i * 2)) * 0.001f;
    average_cell_voltage = average_cell_voltage + cell_voltage;
    if (cell_voltage < min_cell_voltage) {
      min_cell_voltage = cell_voltage;
      min_voltage_cell = i + 1;
    }
    if (cell_voltage > max_cell_voltage) {
      max_cell_voltage = cell_voltage;
      max_voltage_cell = i + 1;
    }
    this->publish_state_(this->cells_[i].cell_voltage_sensor_, cell_voltage);
  }
  average_cell_voltage = average_cell_voltage / cells;

  this->publish_state_(this->min_cell_voltage_sensor_, min_cell_voltage);
  this->publish_state_(this->max_cell_voltage_sensor_, max_cell_voltage);
  this->publish_state_(this->max_voltage_cell_sensor_, (float) max_voltage_cell);
  this->publish_state_(this->min_voltage_cell_sensor_, (float) min_voltage_cell);
  this->publish_state_(this->delta_cell_voltage_sensor_, max_cell_voltage - min_cell_voltage);
  this->publish_state_(this->average_cell_voltage_sensor_