mesh: Rename "node" to "bnode"

[pmav99]

@brey would it be possible to rename node to bnode or something?

node is kind of too generic. And it does conflict with e.g. thalassa naming scheme (probably with other naming schemes too). I don't really know what other changes we would need to make though.

[brey]

Can you propose one name. It shouldn't be difficult to do. I can include it in the next PR.

[tomsail]

node should be renamed boundary_node_connectivity

http://ugrid-conventions.github.io/ugrid-conventions/conformance/#R114