Add support to dense eigen on GPU

[albertomercurio]

Checklist

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Description

The support to dense eigen solvers was missing. The simple case eigenstates(H_gpu) was not working due to missing methods for to_dense(::Type{CuSparseMatrix...}). That method was called inside the eigen function in order to force type stability on CPU (see JuliaLang/LinearAlgebra.jl#1498). However, this just gives a weak Union type on Julia > v1.10, which is not a huge problem. However, this creates a type instability in Julia v1.10, but I think it can be kept like this since it is a problem of that version. So I have decided to remove this forcing type conversion and keep the code simpler.

Since this now gives a Union type, while the sparse solver doesn't, I have decided to use Val for the sparse keyword argument, such that the sparse = Val(true) case doesn't return the Union type.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.92%. Comparing base (3077816) to head (95c39d0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #609      +/-   ##
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- Coverage   93.34%   92.92%   -0.42%     
==========================================
  Files          50       50              
  Lines        3621     3620       -1     
==========================================
- Hits         3380     3364      -16     
- Misses        241      256      +15     

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