Updates residual and jacobian functions

[jmartfrut]

Updates the residual and jacobian functions in the README to reflect changes in the physics model.

This modification aims to provide a more accurate representation of the electromechanical coupling, enhancing the usability of the README as a reference for users.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	d02415f...	main / d02415f...
Constitutive models/Visco-elastic Ψ	0.0504 ± 0.01 ms	0.0507 ± 0.01 ms	0.993 ± 0.29
Constitutive models/Visco-elastic ∂Ψu	0.0667 ± 0.0042 ms	0.0665 ± 0.0036 ms	1 ± 0.084
Constitutive models/Visco-elastic ∂Ψuu	0.116 ± 0.016 ms	0.117 ± 0.016 ms	0.997 ± 0.19
Simulations/StaticMechanicalDirichlet	0.144 ± 0.0074 s	0.143 ± 0.01 s	1.01 ± 0.088
Simulations/StaticMechanicalNeumann	0.132 ± 0.014 s	0.131 ± 0.013 s	1.01 ± 0.15
Simulations/ViscoElastic	8.47 s	8.54 s	0.992
Tensor algebra/Cofactor	0.07 ± 0.001 μs	0.07 ± 0.01 μs	1 ± 0.14
Tensor algebra/Det(A)Inv(A')	0.14 ± 0.01 μs	0.131 ± 0.01 μs	1.07 ± 0.11
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	2.73 ± 0.06 s	2.71 ± 0.074 s	1 ± 0.035

Memory benchmarks

	main	d02415f...	main / d02415f...
Constitutive models/Visco-elastic Ψ	1.25 k allocs: 0.0789 MB	1.25 k allocs: 0.0789 MB	1
Constitutive models/Visco-elastic ∂Ψu	1.37 k allocs: 0.0911 MB	1.37 k allocs: 0.0911 MB	1
Constitutive models/Visco-elastic ∂Ψuu	2.33 k allocs: 0.158 MB	2.33 k allocs: 0.158 MB	1
Simulations/StaticMechanicalDirichlet	1.61 M allocs: 0.115 GB	1.61 M allocs: 0.115 GB	1
Simulations/StaticMechanicalNeumann	1.48 M allocs: 0.0933 GB	1.48 M allocs: 0.0933 GB	1
Simulations/ViscoElastic	0.129 G allocs: 8.3 GB	0.129 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	0.149 k allocs: 11.1 kB	0.149 k allocs: 11.1 kB	1