#9 Unique ID is not supported by EMF

[Charles Fulnecky]

The EMF framework does not prevent the users from
entering invalid or non-unique id strings. Invalid ID
strings generate an unhandled exception:

org.eclipse.emf.ecore.xml.type.InvalidDatatypeValueExce
ption: Invalid NCName value:

Proposed fix:
Modify UseCase and Requirement constructors to
automatically generate a valid ID and make the the ID
element non-editable.