Implementation of partial struxml workflow (.str as base model)

[andosca]

• [ ] #609
• [ ] #520 (should be fixed when this is implemented)

[andosca]

Duplicate of #609

[Marco-Pellegrino]

additioanl request https://femdesign-api.discourse.group/t/editing-an-existing-model-without-closing-and-reopening-it/208