Cofactor function

Reminder for me to factor out the cofactor matrix computation https://github.com/Ferrite-FEM/Tensors.jl/blob/258540f0879d8e4af67700b135ffe8806eaa516e/src/math_ops.jl#L80-L132 into a separate function together with its analytical gradient. We need this one frequently for integration over the reference domain to connect with the normal in the deformed domain (see Nansons formula). Needs to check for performance regressions.

Users will for now probably use the identity $cof(F) = det(F) F^{-T}$, which is arguably less efficient.