Added tree-walk suffixes to PartValue.

With these new suffixes you can walk the parts of a vessel with
tree thinking:

Note: Because there's still no "null" in KOS this is not quite entirely usable yet because walking up the tree you can't tell what happens when PARENT is null.  But you can deal with children that are "null" because that returns an empty ListValue of zero items.

New suffixes:

VesselTarget :ROOTPART
PartValue :PARENT
PartValue :CHILDREN