Add support for 3d EOSes for Spec initial data

[ctrandrsn]

Proposed changes

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Further comments

[nilsdeppe]

LGTM! If you, in this PR you can add a commit that modifies line 13 of src/Evolution/Systems/GrMhd/GhValenciaDivClean/AllSolutions.hpp to also have grmhd::AnalyticData::SpecInitialData<3> in it :) I think that's all that's needed to then actually read the initial data!