Make steadystate function almost compatible with GPU arrays

[albertomercurio]

I make the steadystate function GPU-compatible.

I now create the matrix and the vector for the linear system, depending on the type of the input liouvillian.

Currently, the only feature not yet supported by GPUs is A \ b, but it might be implemented in the future.

This could be solved using iterative algorithms of LinearSolve.jl, which will be implemented in the next PRs related to #81.

To make this, I needed to import the ArrayInterface.jl package, in order to use the allowed_setindex! function. This function allows to perform scalar indexing in a controlled way, that is exactly needed in this case.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.04%. Comparing base (f0a6e1c) to head (9757448).

Files	Patch %	Lines
src/general_functions.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
- Coverage   91.04%   91.04%   -0.01%     
==========================================
  Files          21       21              
  Lines        1754     1765      +11     
==========================================
+ Hits         1597     1607      +10     
- Misses        157      158       +1     

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