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

... and also comments.

The default impl., which is not doing anything, is error prone,
because passing an object of not matrix type, e.g. std::unique_ptr<PETScVector>,
the default impl. will be taken.

Used diagonal of tensor and fixed norm

Move the test to Tests/MathLib and extract MathLib::tensorToKelvin for completness.

This is giving a shortened version of the matrix
in a vector form.

Also specify all of the template arguments as needed for the MSVC
compiler.

Clang format run on near-the-header lines.

Happy New Year

This fixes a clang warning.