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