In this commit, we provide a complete revision and extension of support for
hypertreegrid objects and filters, hereby entirely superseding the first
versions thereof which we developed in 2011-2012.
When compared to those obtained by constructing an intermediate
unstructured mesh with fully described connectivity, this new structure
demonstrates a gain of at least 80% in terms of memory footprint,
with a better rendering while retaining similar execution speed.

All ancillary structures such as cursors and supercursors have also
been entirely refactored, for the sake of performance as well as of
ease of maintenance. Existing native filters, i.e., that operate
directly on hypertreegrid objects, have been revised accordingly.
In addition, several new native filters have been added, together with
a large set of non-regression tests and baseline images.

This commit also implements an adaptive approach in order to accelerate the
rendering of 2-dimensional hypertreegrids, hereby solving the problem posed
by the loss of interactivity that occurs when dealing with large and/or
deeply refined meshes. Specifically, view parameters are taken into account,
in order to: on one hand, avoid creating surface elements that are
outside of  the view area; on the other hand, utilize level-of-detail
properties to cull those cells that are deemed too small to be visible
with respect to the given view parameters. This adaptive approach typically
results in a massive increase in rendering performance.

Please refer to the following articles for a comprehensive description:
https://arxiv.org/abs/1702.04852
https://arxiv.org/abs/1703.00212