Extends TrialFESpace to support ConstantFESpace

[jmartfrut]

Modifies the TrialFESpace constructor for MultiFieldFESpace to allow for ConstantFESpace in the vector of trial spaces.

This enables the use of constant functions as trial functions within a multi-field finite element space.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	5864c58...	main / 5864c58...
Constitutive models/Visco-elastic Ψ	0.0494 ± 0.027 ms	0.049 ± 0.027 ms	1.01 ± 0.78
Constitutive models/Visco-elastic ∂Ψu	0.0641 ± 0.013 ms	0.0642 ± 0.0061 ms	0.999 ± 0.22
Constitutive models/Visco-elastic ∂Ψuu	0.113 ± 0.015 ms	0.113 ± 0.015 ms	1 ± 0.18
Simulations/StaticMechanicalDirichlet	0.143 ± 0.0069 s	0.146 ± 0.0093 s	0.985 ± 0.079
Simulations/StaticMechanicalNeumann	0.134 ± 0.013 s	0.13 ± 0.013 s	1.03 ± 0.14
Simulations/ViscoElastic	20.8 s	20.9 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')	0.13 ± 0.001 μs	0.131 ± 0.01 μs	0.992 ± 0.076
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	2.45 ± 0.018 s	2.45 ± 0.019 s	0.998 ± 0.011

Memory benchmarks

	main	5864c58...	main / 5864c58...
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	1.36 k allocs: 0.0898 MB	1.36 k allocs: 0.0898 MB	1
Constitutive models/Visco-elastic ∂Ψuu	2.32 k allocs: 0.155 MB	2.32 k allocs: 0.155 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	0.343 G allocs: 22 GB	0.343 G allocs: 22 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