NonlinearLeastSquaresProblem Solvers: Levenberg & Gauss Newton

[avik-pal]

• [x] Wait on https://github.com/SciML/SciMLBase.jl/pull/500
• [x] Wait on https://github.com/SciML/DiffEqBase.jl/pull/933
• [x] Levenberg
• [x] Gauss Newton
• [x] Tests
• [x] Look into how convergence checks are implemented in other libraries

[codecov[bot]]

Codecov Report

Merging #218 (9a579c8) into master (cc04a70) will increase coverage by 93.75%. The diff coverage is 85.58%.

@@             Coverage Diff             @@
##           master     #218       +/-   ##
===========================================
+ Coverage    0.00%   93.75%   +93.75%     
===========================================
  Files           8        9        +1     
  Lines         775      832       +57     
===========================================
+ Hits            0      780      +780     
+ Misses        775       52      -723     

Files	Coverage Δ
src/NonlinearSolve.jl	89.47% <100.00%> (+89.47%)	:arrow_up:
src/levenberg.jl	98.42% <100.00%> (+98.42%)	:arrow_up:
src/raphson.jl	100.00% <ø> (+100.00%)	:arrow_up:
src/trustRegion.jl	99.35% <100.00%> (+99.35%)	:arrow_up:
src/utils.jl	78.40% <100.00%> (+78.40%)	:arrow_up:
src/jacobian.jl	88.52% <88.88%> (+88.52%)	:arrow_up:
src/gaussnewton.jl	75.00% <75.00%> (ø)

... and 2 files with indirect coverage changes

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[avik-pal]

This is ready for merge!