Use Optim's iterator interface

[tkf]

TODO:

• Wait Add iterator interface JuliaNLSolvers/Optim.jl#745
• Merge Use Run.jl #3 (or wait for Optim.jl release)
• Revert "Use my fork of Optim.jl" 6fcca92

[codecov-io]

Codecov Report

Merging #4 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #4   +/-   ##
=======================================
  Coverage   87.17%   87.17%           
=======================================
  Files           4        3    -1     
  Lines          39       39           
=======================================
  Hits           34       34           
  Misses          5        5

Impacted Files	Coverage Δ
src/solvers.jl	100% <100%> (ø)	⬆️
src/SteadyStateFit.jl

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