Geometry_Engine: Addition of more geometry conversions

#### Description:

We're duplicating the methods below in other toolkits that convert between various forms of geometry and it'd be good to have them in a more accessible place. Some of these may already exist in some form elsewhere (and if that's the case please let me know where!), but we've got some methods in another toolkit that could be transplanted if not.

- `BH.Engine.Geometry.Convert.ToPolyline`
- `Mesh` - a set of closed polylines describing the faces of the mesh
- `Extrusion` - a closed polyline of the external edges of the extrusion
- `ISurface` - a closed polyline of the external/internal edges of the surface
- `Loft` - a closed polyline of the external edges of the loft
- `NurbsSurface` - a closed polyline of the external edges of the nurbs surface
- `PlanarSurface` - a closed polyline of the external/internal boundary of the planar surface
- `PolySurface` - a set of closed polylines describing the external/internal edges of the component ISurfaces

I'm finding that passing geometry between tools without API's requires reducing our objects down to their vertex representations, and extending these objects with a `ToPolyLine` method would greatly reduce the length of lines being written for geometry interop!