Simplify cast from IGH_Goo to GH_Goo<>

[cdriesler]

The current version of the NodePen Speckle converter has a wonderfully simple overload for ConvertToSpeckle that takes a GH_Goo<T> of any kind and converts it to the Speckle geometry that we need.

Unfortunately, the thing that would like to call it only has type-safe access to an IGH_Goo object. So close! And yet, not close enough to invoke the simple generic convert method.

As a result, I had to write this beauty as a little bridge. There has to be a better, terse, and more generic way to do this!