Fix and test for issue 100

[lkdvos]

This is a more conservative approach to using the multithreaded kernels, which could even be reenabled by users by overloading the _use_multithreaded_array_kernel.

Fixes #100

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.49%. Comparing base (1b175b7) to head (e2d0299).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   85.37%   85.49%   +0.11%     
==========================================
  Files          30       30              
  Lines        3268     3274       +6     
==========================================
+ Hits         2790     2799       +9     
+ Misses        478      475       -3     

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[Jutho]

Maybe, in order to avoid duplication of cases:

_use_multithreaded_array_kernel(y) = _use_multithreaded_array_kernel(typeof(y))
_use_multithreaded_array_kernel(::Type) = false
function _use_multithreaded_array_kernel(::Type{<:Array{T}}) where {T<:Number}
     return isbitstype(T) && get_num_threads() > 1
 end
 function _use_multithreaded_array_kernel(::Type{<:OrthonormalBasis{T}}) where {T}
     return _use_multithreaded_array_kernel(T)
 end

[Jutho]

Looks good. Could you also check if this is the only reason why GPUArraysCore.jl was introduced as dependency, and if so, remove it from Project.toml?

[Jutho]

With the new functionality of the other PR, I would probably go to 0.9. Unless you need this as quickly as possible (I guess the other one would take a few more changes and one more CI run) and want to already tag this right now. I can then also change to 0.9 after merging #105 .

[lkdvos]

I think it would be convenient for me if it was 0.8.3, since the RecursiveVec thing broke things... In the end, it does not matter too much, I changed this here to test it out with MPSKit

[Jutho]

Feel free to merge when you see fit.