Reduce allocations and improve performance in syevr!

[BioTurboNick]

Profiling code that made heavy use of eigen, creating the final arrays here was a hot spot.

I imagine the @view is going to get a frown... it's strictly reducing the second dimension so in theory it could be resized in-place, but I don't know of a way to do that? EDIT: No longer using a view

const AAA = [0.9124359407098924 0.9654684977248279 0.9237069034235668 0.4479855053248103; 0.6313189619311209 0.039044817699327994 0.21147802367801194 0.2242156559761067; 0.9346485775560693 0.7639560598623737 0.30303677867123346 0.33048614822558353; 0.5034773545892783 0.21614932409092613 0.37837816060665885 0.6122920572414832]

function f(a)
      for i = 1:1000
           eigen(Symmetric(a))
      end
end

Before:

After:

[andreasnoack]

It would he good to include the before and after profiling results here to give an idea about the impact

[BioTurboNick]

Thanks, added. Figured I should also do Hamiltonian too because it's the exact same situation.

Looks like there are other spots that could benefit from this as well.

EDIT: found an alternative to @view: initialize Z as a vector and then reshape(resize!()) at the end; extra allocations, but still fewer than the original, and similar performance to the view version.