Added double contraction operator :

[miguelmaso]

The double contraction operator A:A is optimized and faster than tr(A'·A). After this PR, a lot of replacements should be made in the constitutive models.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	552883b...	main / 552883b...
Constitutive models/Visco-elastic Ψ	0.0496 ± 0.029 ms	0.0498 ± 0.028 ms	0.996 ± 0.81
Constitutive models/Visco-elastic ∂Ψu	0.0639 ± 0.0047 ms	0.0644 ± 0.0075 ms	0.993 ± 0.14
Constitutive models/Visco-elastic ∂Ψuu	0.111 ± 0.015 ms	0.112 ± 0.015 ms	0.99 ± 0.19
Simulations/StaticMechanicalDirichlet	0.14 ± 0.0065 s	0.148 ± 0.0091 s	0.948 ± 0.073
Simulations/StaticMechanicalNeumann	0.125 ± 0.0096 s	0.126 ± 0.012 s	0.993 ± 0.12
Simulations/ViscoElastic	8.07 s	8.43 s	0.958
Tensor algebra/Cofactor	0.07 ± 0.01 μs	0.061 ± 0.01 μs	1.15 ± 0.25
Tensor algebra/Det(A)Inv(A')	0.13 ± 0.01 μs	0.14 ± 0.011 μs	0.929 ± 0.1
Tensor algebra/δδ_λ_2d	30 ± 0 ns	30 ± 0 ns	1 ± 0
Tensor algebra/δδ_μ_2d	30 ± 0 ns	30 ± 0 ns	1 ± 0
time_to_load	2.41 ± 0.001 s	2.46 ± 0.02 s	0.977 ± 0.008

Memory benchmarks

	main	552883b...	main / 552883b...
Constitutive models/Visco-elastic Ψ	1.25 k allocs: 0.0794 MB	1.25 k allocs: 0.0794 MB	1
Constitutive models/Visco-elastic ∂Ψu	1.37 k allocs: 0.0917 MB	1.37 k allocs: 0.0917 MB	1
Constitutive models/Visco-elastic ∂Ψuu	2.31 k allocs: 0.158 MB	2.31 k allocs: 0.158 MB	1
Simulations/StaticMechanicalDirichlet	1.59 M allocs: 0.114 GB	1.59 M allocs: 0.114 GB	1
Simulations/StaticMechanicalNeumann	1.41 M allocs: 0.0901 GB	1.41 M allocs: 0.0901 GB	1
Simulations/ViscoElastic	0.128 G allocs: 8.3 GB	0.128 G allocs: 8.3 GB	1
Tensor algebra/Cofactor	1 allocs: 0.0781 kB	1 allocs: 0.0781 kB	1
Tensor algebra/Det(A)Inv(A')	4 allocs: 0.25 kB	4 allocs: 0.25 kB	1
Tensor algebra/δδ_λ_2d	0 allocs: 0 B	0 allocs: 0 B
Tensor algebra/δδ_μ_2d	0 allocs: 0 B	0 allocs: 0 B
time_to_load	0.159 k allocs: 11.6 kB	0.149 k allocs: 11.1 kB	1.04

[miguelmaso]

Duplicated. inner must be used