TreeNode data dict

[Licini]

          for the tree node there are two options

1. keep the attributes dict in the data dict and convert the node explicitly to/from data
2. don't include the attributes dict in the data dict and let the encoder/decoder handle to/from (then the attributes are included implicitly)

the file size of 1. is much smaller and is similar to what happens to nodes in a graph and vertices in a mesh. however, there the behaviour is not so explicit since the nodes and vertices are not separate objects.

if a tree node is not meant to exist on its own (like graph nodes and mesh vertices), perhaps we should make sure that they cannot be converted to/from data independently?

Originally posted by @tomvanmele in #1247 (comment)