Fix segfaults created by recently lowered value of MAX_SONS

Previously, the value of MAX_SONS for 2d grids was larger than
necessary. Some parts of the code relied on this: They assumed
that there would always be a nullptr entry that would mark
the end of the array.

Commit changed this:
The number MAX_SONS was reduced for 2d grids to 4, which is the
actual maximum number of sons a 2d element may have.  As a
consequence, some loops now have to be told explicitly to not
loop farther than MAX_SONS entries.

This fixes #62