SYSML2 — UML4SysML::Class should be mapped to ItemDefinition

UML4SysML::Class is currently mapped to OccurrenceDefinition, but should be ItemDefinition.

Change the mapping target of UML4SysML::Class to ItemDefinition

The intent of ItemDefinition is to have a class-like construct. However, in SysML v1, a Behavior can specialize a Class, for example, an Activity. Therefore, a Class must be mapped to OccurrenceDefinition to allow a specialization between the target elements in SysML v2, for example, an ActionDefinition (mapping target of Activity) and an OccurrenceDefinition (mapping target of Class).