add energy threshold diff mode

by setting enabled the second threshold and the difference mode one get get an image = threshold2 - threshold1. So add a new attribute energy_threshold2 and a new command set_energy_threshold_diff_mode True or False

To set a specific energy as second threshold (in the example 52000 eV):

lid17medg> curl -X PUT http://lid17eiger2dcu.esrf.fr/detector/api/1.8.0/config/threshold/2/energy -H "Content-Type: application/json" -d '{"value": 52000}'

lid17medg> curl -X PUT http://lid17eiger2dcu.esrf.fr/detector/api/1.8.0/config/threshold/2/mode -H "Content-Type: application/json" -d '{"value": "enabled"}'

lid17medg> curl -X PUT http://lid17eiger2dcu.esrf.fr/detector/api/1.8.0/config/threshold/difference/mode -H "Content-Type: application/json" -d '{"value": "enabled"}'