Change the indexing of HyperTree and remove dead code & vars.

HT are now indexed in such a way that there is no more
dead index.
HTGSource in quadric mode use the GlobalIndexStart mode to
avoid the storage of the local to global node id per tree
(contrary to descriptor/mask sources which use a BSF global
indexing mode, quadric nodes are numberered on a per-tree
basis).

Also remove some no more used functions and vars.

Change-Id: Ie5fe5f267a22706334d883d330b8aa9bbb3926e2