added Is Null? node
added basic null value error notification
Merge branch 'master' of github.com:ikeough/Dynamo
Add units test for modulo, negativity, construction with negatives.
Cleanup.
Add tests for conversion extensions.
Rename Unwrap.
Rename UnwrapToSIUnit. Globally convert SIUnits to numbers in eval.
Return lengths where applicable.
Convert units coming out of revit nodes to SI.
Avoid a name collision with solid volume.
Rename LibG surface area to avoid name clash.