Previously only used in the linear elastic orthotropic
model, but the transformation itself is more general
and is being used in other (not yet available) code
parts.

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