Mesh evaluate node / Mesh Expression

Created by: portnov

Initial idea. To create useful meshes, it would be nice to get a mesh modelled "by hand", and parametrize it.

Proposed implementation. Step 1: get selected mesh object and store it, for example, in JSON format (dict with "vertices", "edges" and "faces" keys). Step 2: replace some numbers in "vertices" list with variable names. Step 3: load resulting JSON and plug variable values as node inputs.

Initial implementation is in https://github.com/portnov/sverchok/tree/mesh_from_json.