Implement OSCAL models according to RMF

[ronaldtse]

Starting from the left:

1. OSCAL Catalog: Incorporate all functionality of convert2oscalyaml.rb script into gem #1, Fully parse OSCAL for the metanorma-plugin-oscal gem #2
2. OSCAL Profile: Implement Profile model #8
3. OSCAL Component definitions: Implement Component model #10
4. OSCAL System Security Plan (SSP) Implement System Security Plan model #9
5. OSCAL Assessment Plan (AP)
6. OSCAL Assessment Results (AR)
7. OSCAL Plans of Actions and Milestones (POAM)
8. OSCAL ConMon Plan (same as AP model)
9. OSCAL ConMon Results (same as AR model)