Add χt parameter to HardMagnetic models

[jmartfrut]

Introduce a new susceptibility parameter χt to decouple torque-related calculations from χe. The parameter defaults to the value of χe if not explicitly provided, maintaining backward compatibility.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	d522847...	main / d522847...
Constitutive models/Visco-elastic Ψ	0.0548 ± 0.034 ms	0.0542 ± 0.033 ms	1.01 ± 0.88
Constitutive models/Visco-elastic ∂Ψu	0.0605 ± 0.0026 ms	0.0602 ± 0.0027 ms	1.01 ± 0.062
Constitutive models/Visco-elastic ∂Ψuu	0.112 ± 0.016 ms	0.113 ± 0.017 ms	0.99 ± 0.21
Simulations/StaticMechanicalDirichlet	0.141 ± 0.011 s	0.142 ± 0.015 s	0.998 ± 0.13
Simulations/StaticMechanicalNeumann	0.132 ± 0.02 s	0.134 ± 0.018 s	0.982 ± 0.2
Simulations/ViscoElastic	20.3 s	20.4 s	0.995
Tensor algebra/Cofactor	0.07 ± 0 μs	0.07 ± 0 μs	1 ± 0
Tensor algebra/Det(A)Inv(A')	0.131 ± 0.011 μs	0.13 ± 0.01 μs	1.01 ± 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	2.64 ± 0.027 s	2.66 ± 0.038 s	0.991 ± 0.017

Memory benchmarks

	main	d522847...	main / d522847...
Constitutive models/Visco-elastic Ψ	1.24 k allocs: 0.0777 MB	1.24 k allocs: 0.0777 MB	1
Constitutive models/Visco-elastic ∂Ψu	1.29 k allocs: 0.0826 MB	1.29 k allocs: 0.0826 MB	1
Constitutive models/Visco-elastic ∂Ψuu	2.25 k allocs: 0.149 MB	2.25 k allocs: 0.149 MB	1
Simulations/StaticMechanicalDirichlet	1.51 M allocs: 0.108 GB	1.51 M allocs: 0.108 GB	1
Simulations/StaticMechanicalNeumann	1.48 M allocs: 0.093 GB	1.48 M allocs: 0.093 GB	1
Simulations/ViscoElastic	0.326 G allocs: 20.5 GB	0.326 G allocs: 20.5 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