The `bool uncaught_exception()' is deprecated since c++17
and is replaced by `int uncaught_exceptions()'.

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

This potentially saves a second copy compared to using const
reference. See Effective Modern C++, Item 41.

The access now must be done through some non-member functions.

Also checks are made publicly callable

const + mutable enable use as temporary variables
move assignment complements move construction