DiscreteMeasurement id attribute has to be optional

[gaouizer]

Describe the current behavior

DiscreteMeasurement is expected to have an id attribute whereas it is an optional attribute for the discrete_measurement (dm) extension. Thus, when trying to open an iidm file where there are dms without ids with powsybl::iidm::Network::readXml an XmlStreamException is thrown (Attribute id does not exists").

Describe the expected behavior

powsybl::iidm::Network::readXml should be able to read iidm files where there is no id attribute in dms

Describe the steps

No response

Environment

No response

Relevant Log Output

No response

Extra Information

There are several places in the corde (e.g in powsybl::iidm::extensions::iidm::DiscreteMeasurements or in powsybl::iidm::extensions::iidm::DiscreteMeasurementAdder classes) where the logic is based on the existence of this attribute, fully solving that bug would require a bit more that solving the readXml wrong behaviour.