This change makes it possible to use interface
function Element::getNode() in Element::addNeighbor
function.

This remove one dependencies on _nodes pointer C array.
If all dependencies are removed the memory for storing
the nodes can be in a std::array.

The getContent() method is used only on very few
places. Especially, it isn't used in the computation
of any THMC processes. So the content doesn't need
to be stored which saves memory. Furthermore, the time
time for reading / initializing a mesh is reduced.

In a hex mesh with 100x100x100 elements this saves
2.6% of storage and circa 25% time.

This mv one dependency from _neighbors from the
base class Element to the class TemplateElement.

This simplifies most of the code.

Change of the interface using structured bindings improves code quality.