Add `get_nnzh` function

[tmigot]

Needed for #83 and fix the docstring as suggested in #86

[github-actions[bot]]

Package name	latest	stable
CaNNOLeS.jl
DCISolver.jl
DerivativeFreeSolvers.jl
JSOSolvers.jl
NLPModelsIpopt.jl
OptimizationProblems.jl
Percival.jl
QuadraticModels.jl
SolverBenchmark.jl
SolverTools.jl

[codecov[bot]]

Codecov Report

Base: 99.42% // Head: 99.43% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (af5daa1) compared to base (b5b436f). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #87 +/- ## ======================================= Coverage 99.42% 99.43% ======================================= Files 7 7 Lines 701 711 +10 ======================================= + Hits 697 707 +10 Misses 4 4 ``` | [Impacted Files](https://codecov.io/gh/JuliaSmoothOptimizers/ADNLPModels.jl/pull/87?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers) | Coverage Δ | | |---|---|---| | [src/ad.jl](https://codecov.io/gh/JuliaSmoothOptimizers/ADNLPModels.jl/pull/87/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers#diff-c3JjL2FkLmps) | `95.83% <100.00%> (+0.37%)` | :arrow_up: | | [src/nlp.jl](https://codecov.io/gh/JuliaSmoothOptimizers/ADNLPModels.jl/pull/87/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers#diff-c3JjL25scC5qbA==) | `100.00% <100.00%> (ø)` | | | [src/nls.jl](https://codecov.io/gh/JuliaSmoothOptimizers/ADNLPModels.jl/pull/87/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers#diff-c3JjL25scy5qbA==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.