Remove allocations for Brownden in grad! and hess_structure!

[tmigot]

#60

  Problem name: BROWNDEN_manual
                   obj: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0.0
              hess_op!: █⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 2304.0
           hess_coord!: ████████████████████ 50976.0
                 grad!: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0.0
       hess_structure!: ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ 0.0
                hprod!: █████████████⋅⋅⋅⋅⋅⋅⋅ 31360.0
         hess_op_prod!: █████████████⋅⋅⋅⋅⋅⋅⋅ 31456.0

[codecov]

Codecov Report

Base: 99.18% // Head: 99.19% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (387ae9f) compared to base (1c92eb8).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   99.18%   99.19%           
=======================================
  Files          25       25           
  Lines        2959     2965    +6     
=======================================
+ Hits         2935     2941    +6     
  Misses         24       24           

Impacted Files	Coverage Δ
src/nlp/problems/brownden.jl	100.00% <100.00%> (ø)

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[github-actions]

Package name	latest	stable
ADNLPModels.jl
AmplNLReader.jl
CUTEst.jl
CaNNOLeS.jl
DCISolver.jl
JSOSolvers.jl
LLSModels.jl
NLPModelsIpopt.jl
NLPModelsJuMP.jl
PDENLPModels.jl
Percival.jl
QuadraticModels.jl
SolverBenchmark.jl
SolverTools.jl