Implementation
setBaseAdjustmentPositive()
https://github.com/magento/magento2/blob/2.1.5/app/code/Magento/Sales/Model/Order.php#L3935-L3941
setBaseAdjustmentNegative()
https://github.com/magento/magento2/blob/2.1.5/app/code/Magento/Sales/Model/Order.php#L3927-L3933
getBaseAdjustmentPositive()
https://github.com/magento/magento2/blob/2.1.5/app/code/Magento/Sales/Model/Order.php#L2047-L2055
getBaseAdjustmentNegative()
https://github.com/magento/magento2/blob/2.1.5/app/code/Magento/Sales/Model/Order.php#L2037-L2045