Thread-safety fixes in bands

[pablosanjose]

There were some thread safety bugs in multithreaded bands

• Arpack segfaults if it is used with threads because the package is currently non thread-safe (Segmentation fault when using Arpack and Threads.@threads JuliaLinearAlgebra/Arpack.jl#86). We now use a single thread in this case (and any other solver that declares is_thread_safe(::AbstractEigenSolver) = false
• There was a bug in apply(solver, h, ...) whereby all threads would end up writing on the same Bloch matrix. Now fixed.
• We should never use multithreading with bands(hf::Function, ...), because hf may very well not be thread-safe

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1613b41) 92.54% compared to head (6f035f8) 92.55%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #219   +/-   ##
=======================================
  Coverage   92.54%   92.55%           
=======================================
  Files          34       34           
  Lines        5443     5450    +7     
=======================================
+ Hits         5037     5044    +7     
  Misses        406      406           

Files	Coverage Δ
src/apply.jl	92.34% <100.00%> (ø)
src/solvers/eigen.jl	97.72% <100.00%> (+0.10%)	⬆️
src/bands.jl	97.66% <91.66%> (+0.02%)	⬆️

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