Faster bands: do set_blas_threads(1) before diagonalization, then restore

[pablosanjose]

The diagonalization step in bands is multithreaded over momenta. BLAS is also multithreaded by default. But actual diagonalization of matrices is more difficult to multithread than the loop over momenta, so it is much faster to switch to 1 single BLAS thread for the diagonalization step in bands, and then return to whatever it was before. The difference is specially dramatic using EigenSolvers.LinearAlgebra, to the point in which it is the fastest method (with multithreading) up to pretty large unit cells (like here, with 200 orbitals, 8 threads)

Master:

➜  ~ julia -t 8 -e "using Quantica, LinearAlgebra; h = HP.graphene() |> supercell(10); s=subdiv(0,2pi,50); @time bands(h, s, s);"
Step 1 - Diagonalizing: 100%|██████████████████████████████████| Time: 0:01:55
Step 2 - Knitting: 100%|███████████████████████████████████████| Time: 0:00:06
126.732104 seconds (26.83 M allocations: 11.157 GiB, 0.36% gc time, 4.15% compilation time)

This PR:

➜  ~ julia -t 8 -e "using Quantica, LinearAlgebra; h = HP.graphene() |> supercell(10); s=subdiv(0,2pi,50); @time bands(h, s, s);"
Step 1 - Diagonalizing: 100%|██████████████████████████████████| Time: 0:00:04
Step 2 - Knitting: 100%|███████████████████████████████████████| Time: 0:00:05
 15.468912 seconds (26.88 M allocations: 11.161 GiB, 3.01% gc time, 39.75% compilation time)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (333ffd8) 92.59% compared to head (97d84d0) 92.60%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #226   +/-   ##
=======================================
  Coverage   92.59%   92.60%           
=======================================
  Files          34       34           
  Lines        5457     5460    +3     
=======================================
+ Hits         5053     5056    +3     
  Misses        404      404           

Files	Coverage Δ
src/bands.jl	97.68% <100.00%> (+0.01%)	⬆️

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