Reenable StructuralIdentifiability tests and tutorial once fixed

[isaacsas]

#1070 disabled StructuralIdentifiability tests and added a warning in the docs about it having issues. Once they are resolved we should reenable the tests.

[isaacsas]

To record what has been investigated here, @TorkelE pointed out on Slack this seems to completely hang Julia:

function testSI()
    t = ModelingToolkit.t_nounits
    D = ModelingToolkit.D_nounits
    @variables X(t)
    @parameters p d
    eqs = [
        D(X) ~ p - d*X
    ]
    @mtkbuild osys = ODESystem(eqs, t)
    measured_quantities = [X]    
    funcs_to_check = [osys.p]

    SI.assess_identifiability(osys; measured_quantities, funcs_to_check)
end

Changing to funcs_to_check = [p] seemed to fix the issue. Note I had reported on Slack that removing logging fixed the issue for me, but that doesn't seem to be true currently (so perhaps it was from a polluted environment in some way).

[isaacsas]

CC: SciML/StructuralIdentifiability.jl#360

[isaacsas]

Docs and extension are fully disabled now via: #1074 (review) so that will need to be reverted when it starts working again.