changes to `nb` objects and use of `spdep::n.comp.nb`

[rsbivand]

In r-spatial/spdep#160 and r-spatial/spdep#161, changes are made to the use of n.comp.nb. It seems now more important to move checks of the possible existence of disjoint subgraphs to the creation and modification of nb neighbour objects, adding these to the nb object at creation or modification in spdep. This means that separate checking may be carried out on the "ncomp" attribute of an nb object from spdep version >= 1.3-6 (branch n_comp until r-spatial/spdep#161 is merged).