Remove is_surface() from Residue/ResidueType.

[roccomoretti]

This function is confusing, as it's not talking about the surface of a protein, but mineral surfaces.

The ResidueType property SURFACE isn't used frequently -- for the rare case it's needed, we can use the more involved query protocol, rather than leaving the potentially confusing Residue::is_surface() lying around.

[ajasja]

Looks good to me. (One option would be to rename to is_type_surface())