Example json files for assignment files

[stuarteberg]

The NeuTu repo has a few example json files (and schemas) for various json inputs that NeuTu/neu3 accepts:

https://github.com/janelia-flyem/NeuTu/tree/master/neurolabi/json

but it looks like not all json file types are included there. In particular, we need examples of all assignment (and bookmark) types that NeuTu/neu3 can read.

cc @umayaml @jtpdowns