Updated tests for Mechanical models

[miguelmaso]

Description
The proposed tests are equivalent to the current. However, the tests are more systematic.

Changelog

• The energy definition of LinearElasticity has been updated to be compatible with ForwardDiff
• The functions _δδ_μ_2D... have been removed from TensorAlgebra (physics ≠ algebra)
• A benchmark is added for the new δᵢⱼδₖₗ2D... Given they are compile time, they take the same time than the previous _δδ_μ_2D
• I've detected that HessianRegularization is failing

[jmartfrut]

go back to
@test Ψ(F(∇u)) == 6440.959849358168
@test norm(∂Ψu(F(∇u))) == 52.8548808805944
@test isapprox(norm(∂Ψuu(F(∇u))), 18128.524371074407, rtol=1e-14)

[miguelmaso]

Done