Seperate out the rest of the `Domain` element dispatch into proper folders

[skygering]

Right now, only two of the domain element dispatches are in the correct folders: _get_velocity and _update_boundary. It would be good to move the rest of the domain dispatches into the correct files and document them as a part of the API. So for example, floe_domain_element_interaction! and everything having to do with ghost floes.