switch to nuget package references

[andrewheumann]

This PR switches the local .DLL references to RhinoCommon and Grasshopper to be NuGet package references, to make it easier for other developers to pull, restore, and build the project.

It looks like you're trying to maintain Rhino 5 compatibility, so I've chosen the oldest versions of RhinoCommon + Grasshopper available on nuget, which should be Rhino 5 compatible, and still work fine in Rhino 6 or 7. I do not have Rhino 5, however, so it may be worth testing just to verify.

[arendvw]

Thanks a lot for the update and the introduction into 2020-compatible reference handing!