What's different between init_bulk and init_surf when using a 2D material

[anatowinter]

I want to init a 2D MoS2 nanorribon, but I don't know which kind of initset I should choose,if I want to calculate thermal conduction inplane, I don't think the system is a surface system.

[HuangJiameng]

init_bulk and init_surf is introduced in the document. For what is done in init_surf, you can refer to #927.