clang-tidy modernize-redundant-void-arg check.

Also those where the second and so forth arguments are defaulted.

The MathLib::TemplateVector3 is an exception, because of the many
required changes through the code, which relies on the implicit
conversion from TemplatePoint<T, 3> to TemplateVector3<T>.

clang-tidy '*explicit*' check.

Conversion happened in the constant vectorXi construction.

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.