Add BLAS.get_num_threads

[mcabbott]

Ref JuliaLang/julia#36360

Note that I didn't export this, leaving it as Compat.get_num_threads(), but did import the version from LinearAlgebra when defined.

Also, I copied the new version of set_num_threads() here, rather than calling the existing one.

[martinholters]

Should set_num_threads() also be mentioned in the README and set_num_threads(nothing) be tested (that it doesn't error, at least)? Or why is it included here?

[mcabbott]

I tried to minimally adapt the PR's tests, but I agree that it's odd that set_num_threads(::Nothing) isn't tested, I can add something. The difference from the existing function is that it accepts nothing, and that it prints more warnings. Seemed simpler to copy the whole new implementation than to split it.

[martinholters]

LGTM.

[martinholters]

This is a bit ugly but I can't think of a better way either.

[mcabbott]

Could also dump these & call testing the overall behaviour enough, here.

[martinholters]

But IIUC, some of the code below explicitly invokes code paths not hit by usual CI, so might be worth it. Not sure.

[martinholters]

Oh, that approval was a bit too fast:

WARNING: Compat.Sys is deprecated, use Base.Sys instead.

  likely near /home/travis/build/JuliaLang/Compat.jl/test/runtests.jl:468

[martinholters]

Will merge once CI passes (assuming it doesn't surprise me with new warnings).