New XML HTG V2 for reader and writer

This new format squashes together every arrays which used to be split by
tree. They can still be read by tree, given some new meta information on
the structure of the htg. One can still load an htg with a depth
delimiter.

Squashing the information saves a lot of space in the file. Prior to
that, one XML element was created per tree, which made outrageous large
files on htg with millions of trees.
13 jobs for htg-writer
Status Job ID Name Coverage
  External
passed cdash-buildbot

passed ghostflow-branch-check-master

failed vtk@master-adora-linux-shared-release+cuda+gcc+mpi+optix+python2+tbb+vtkm

00:00:10

failed vtk@master-bigmac-osx-shared-debug+adios2+clang+python2+vtkm

00:26:27

failed vtk@master-duma-linux-shared-release+osmesa+ospray

00:25:35

failed vtk@master-eeloo-linux-shared-release+adios2+extdeps+java+mpi+python3+qt+tbb+vtkm

04:00:18

running vtk@master-eeloo-linux-shared-release+mpi+nogl

776:01:57

failed vtk@master-luigi-linux-shared-release+mpi+python2+tbb

00:00:49

failed vtk@master-mun-windows-shared-release+kits+mpi+python3+qt+tbb

00:55:48

failed vtk@master-mun-windows-shared-release+mpi+python3+tbb+vtkm

00:24:18

failed vtk@master-mun-windows-static-release+mpi+qt+tbb

00:55:21

failed vtk@master-taanab-linux-shared-release+adios2+mpi+python2

00:20:19

failed vtk@master-trey-osx-shared-release+python3+qt

00:59:57