DUI3-38 Clarify and Support valid fallback value types

[clairekuang]

Do not merge - record of Interfacing work done to constrain fallback displayvalue types

Key points:

• New IBasicGeometryType interface implemented by Point, Line, Polyline, Mesh
• IDisplayValue updated to only accept IEnumerable<IBasicGeometryType>
• POC updating classes to new display values - breaking change here for eg classes that were previously not List

Blocking:

• probably will want to remove structural classes, and GH constructors before making this change
• discuss IBase first

[clairekuang]

Freeform elements and DirectShapes currently break the assumption that displayValue lists will only have objects of one type: I believe we are supporting display values that are a mix of curves and meshes in these classes.