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.

Change of the interface using structured bindings improves code quality.

The fmt library happily accepts std::string without
any conversions.

Include spdlog via BaseLib/Logging.h, which
provides macro replacements for
ERR/DBUG/WARN/INFO...

Only in ERR/WARN/INFO/DBUG/OGS_FATAL and similar, not in
formatted output like xml files.

Clang format run on near-the-header lines.

Happy New Year