Rework lower/upper in ParticleSwarm for stability.

[pkofod]

Fixes #1033

[codecov[bot]]

Codecov Report

Merging #1037 (a318cd6) into master (7fa215a) will increase coverage by 0.03%. The diff coverage is 100.00%.

:exclamation: Current head a318cd6 differs from pull request most recent head 695c11f. Consider uploading reports for the commit 695c11f to get more accurate results

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
+ Coverage   85.25%   85.29%   +0.03%     
==========================================
  Files          42       43       +1     
  Lines        3194     3202       +8     
==========================================
+ Hits         2723     2731       +8     
  Misses        471      471              

Impacted Files	Coverage Δ
src/multivariate/optimize/interface.jl	82.07% <100.00%> (+0.94%)	:arrow_up:
...tivariate/solvers/constrained/ipnewton/ipnewton.jl	96.64% <100.00%> (ø)
...olvers/first_order/accelerated_gradient_descent.jl	100.00% <100.00%> (ø)
...e/solvers/first_order/momentum_gradient_descent.jl	100.00% <100.00%> (ø)
src/multivariate/solvers/first_order/ngmres.jl	94.90% <100.00%> (ø)
...ultivariate/solvers/zeroth_order/particle_swarm.jl	98.85% <100.00%> (-0.38%)	:arrow_down:

... and 2 files with indirect coverage changes

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