Extract PVMT and Requirement API from capella.py

[Aurelien-Pin]

Extract PVMT and Requirement API from capella.py in order to have:

• one file with Capella simplified metamodel
• one file with PVMT API
• one file with Requirement add-on API

This will show a good practice of structuring definitions (and show that additional add-ons can be covered by defining new scripts)