Revise rules for regenerating latex

Revise rules for regenerating latex

To ensure that only rubber tree trunk nodes that are part of trees
regenerate, rather than those that are used as building blocks, check
that they are sufficiently close to rubber tree leaves.  This replaces the
older rule that naturally-grown trunks regenerate (regardless of leaves)
and manually-placed trunks don't (even though manually building a tree
otherwise works).  The detection of manually-placed trunks was in any
case broken for users of moretrees, because that mod fails to set the
flag signalling manual placement.

Incidentally also fix a bug that caused rubber tree branches (horizontal
trunk nodes) to turn vertical when regenerating latex.  Rather than set
the complete node structure, only switch the type name, as does the tree
tap when emptying the node.