Fix steepest descent to cache previous param

[DevonMorris]

As discussed in #362

[stefan-k]

Thanks a lot for the quick fix!! This looks good to me, I only have a minor request to make the CI pass.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (ae98919) 93.35% compared to head (71432a6) 93.36%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #363   +/-   ##
=======================================
  Coverage   93.35%   93.36%           
=======================================
  Files         117      117           
  Lines       18803    18819   +16     
=======================================
+ Hits        17554    17570   +16     
  Misses       1249     1249           

Impacted Files	Coverage Δ
...gmin/src/solver/gradientdescent/steepestdescent.rs	95.62% <100.00%> (+0.57%)	⬆️
argmin/src/solver/linesearch/backtracking.rs	99.05% <100.00%> (ø)
argmin/src/solver/linesearch/hagerzhang.rs	73.89% <100.00%> (ø)
argmin/src/solver/linesearch/mod.rs	77.27% <100.00%> (ø)
argmin/src/solver/linesearch/morethuente.rs	86.60% <100.00%> (ø)

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[stefan-k]

Thanks again! :)