Updating the types in Levenberg

[Deltadahl]

Changing the types in levenberg.jl to match the input u0 given by the user as #132 (comment) suggests for trustRegion.jl

[codecov]

Codecov Report

Merging #134 (bcb3338) into master (e839519) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   90.89%   91.00%   +0.10%     
==========================================
  Files           7        7              
  Lines         494      500       +6     
==========================================
+ Hits          449      455       +6     
  Misses         45       45              

Impacted Files	Coverage Δ
src/levenberg.jl	95.86% <100.00%> (+0.17%)	⬆️

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[Deltadahl]

I don't find any place where making the input rational would change anything.
In all cases, I just take the input given by the user or the default values

damping_initial::Real = 1.0,
damping_increase_factor::Real = 2.0,
damping_decrease_factor::Real = 3.0,
finite_diff_step_geodesic::Real = 0.1,
α_geodesic::Real = 0.75,
b_uphill::Real = 1.0,
min_damping_D::AbstractFloat = 1e-8)

and then convert it to the element type of u by x = convert(eltype(u), alg.x) before putting x into cache.