[cleanup] Remove obsolete macros SIZEOF_REF and NULL_REF

Those were only used in the fortran backend, which has been removed.
[cleanup] Remove the fields lb2 and lb3 from the 'element' structs

They were used by the Chaco mesh partitioner, but the Chaco code has
been removed from the UG git tree a long time ago for license reasons.
The lb2 and lb3 fields didn't have any purpose after that, but
apparently this fact went unnoticed.  Now they have been removed,
which should save 8 Bytes per element.
Hardwire the NEW_GRIDCONS_STYLE flag

It has been set ever since 1998.  I don't think we'll ever go back
to the old style.
Set the SideVector VCOUNT fields after load balancing

For each SideVector, the VCOUNT field stores how many elements reference
the SideVector (2 or 1).  The VCOUNT field is apparently not touched
during load balancing at all.  Therefore, if it was set properly before
load balancing it is most likely screwed up afterwards, as elements have
gained/lost neighbor elements.  This leads to strange segfaults much
later.  Most notable was Dune FlySpray issue 810:

https://dune-project.org/flyspray/index.php?do=details&task_id=810

which was hacked around in the UGGrid Dune-UG interface class as early
as 2010, in dune-grid commit 5e870eb04580e3279277ffdad917edf7c4dbae18

This patch here now fixes the issue 'properly', within UG itself.
It doesn't really do anything else than the UGGrid hack: after load balancing
has been completed, rewrite all VCOUNT fields.  After some investigation
I did not see any better way to obtain proper VCOUNT fields.  At least
the code is in the best spot for this now.  Namely it is part of the
method ConstructConsistentGridLevel.
[cleanup] Remove code that parse options for the Chaco load-balancer

The Chaco load balancing code has been remove years ago, but the code that
parses argument strings for Chaco arguments was still there.  This patch
cleans up the code, removing all obsolete load balancer options.