Use namedtuple for default options.

[pkofod]

Fixes #1021

[codecov]

Codecov Report

Merging #1026 (de45d21) into master (7fa215a) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
+ Coverage   85.25%   85.30%   +0.05%     
==========================================
  Files          42       43       +1     
  Lines        3194     3199       +5     
==========================================
+ Hits         2723     2729       +6     
+ Misses        471      470       -1     

Impacted Files	Coverage Δ
src/multivariate/optimize/interface.jl	82.07% <100.00%> (+0.94%)	⬆️
...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)
src/utilities/generic.jl	100.00% <0.00%> (ø)
src/Optim.jl	100.00% <0.00%> (ø)

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