Point3d.Unset should have a special string ovevrride

[goswinr]

When calling ToString() on Point3d.Unset it prints
-1.23432101234321E+308,-1.23432101234321E+308,-1.23432101234321E+308
a string literal like
Point3d.Unset
or
Point3d.Unset (-1.2E+308, -1.2E+308, -1.2E+308)
would be nicer so that it is explicit that this is a special point value.
This helps in debugging too.
at line

rhinocommon/dotnet/opennurbs/opennurbs_point.cs

Line 1872 in 57c3967

public override string ToString()

[goswinr]

I would in general add a type annotation and format any point as
Point3d(1,2,3)
instead of
1,2,3
but i understand that this might be a problem if someone relies on the current behaviour.