Add Support for Capturing Structural Features

[ikiril01]

We've identified that certain existing Capabilities, e.g., Code Compression, are really more structural than behavioral features. As such, we should add a new "Structural Features" construct to MAEC for capturing such entities. This will likely be similar to the existing Capabilities structure, albeit simpler.

[ikiril01]

E.g,

<Structural_Features>
  <Structural_Feature>
    <Name xsi:type="maecVocabs:StructuralFeatureVocab-1.0">code compression</Name>
  </Structural_Feature>
</Structural_Feaures>