[BUG] Use sensible default namespace for new Aspect Models

[atextor]

Is your task related to a problem? Please describe.

• Create a new Aspect Model

Describe the solution you'd like

• What happens: The Aspect is implicitly created in the namespace urn:samm:org.eclipse.esmf:1.0.0
• What should happen
  • I'm asked for the new namespace (see also [Task] Settings Namespaces or: new aspect model in new namespace #243), or
  • I can select one of the existing namespaces to create the Aspect in, or
  • A neutral default is used, such as urn:samm:com.example:1.0.0

Model authors outside the ESMF organization must not use an org.eclipse.esmf namespace! This must not be the default for new models.