Prepare for AdjointFactorization, multiple dispatch

[dkarrasch]

This prepares the ground for JuliaLang/julia#46874. The const definition and the method signatures are harmless for older Julia versions. I also rearranged the convert functions and used multiple dispatch. I don't know why, but the changes fix a broken test on Julia v1.9, but not older.

[codecov]

Codecov Report

Merging #138 (1c9e822) into master (d728a14) will decrease coverage by 64.54%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #138       +/-   ##
==========================================
- Coverage   64.54%   0.00%   -64.55%     
==========================================
  Files          10      10               
  Lines        1224    1221        -3     
==========================================
- Hits          790       0      -790     
- Misses        434    1221      +787     

Impacted Files	Coverage Δ
src/matrix.jl	0.00% <0.00%> (-63.12%)	⬇️
src/tensor.jl	0.00% <0.00%> (-78.08%)	⬇️
src/func.jl	0.00% <0.00%> (-73.65%)	⬇️
src/basic.jl	0.00% <0.00%> (-69.65%)	⬇️
src/utils.jl	0.00% <0.00%> (-66.67%)	⬇️
src/batch.jl	0.00% <0.00%> (-58.63%)	⬇️
src/scalar.jl	0.00% <0.00%> (-54.21%)	⬇️
src/interface.jl	0.00% <0.00%> (-41.38%)	⬇️
src/left.jl	0.00% <0.00%> (-37.50%)	⬇️
... and 1 more

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