`deepcopy` to save population in default trace, instead of reference

[jonathanfischer97]

Default trace! overload was saving population by reference

• My previous PR #702 was actually saving a reference to population in trace["x"], which was not the intention.

  # Overload the trace! function to add the population to the trace prior to calling any user-defined trace! method
  function Evolutionary.trace!(tr, iteration, objfun, state, population,
      method::Evolutionary.AbstractOptimizer, options, curr_time = time())
  dt = Dict{String, Any}()
  dt["time"] = curr_time
  
  # record `x` to store the population. Needed for constructing OptimizationState.
  dt["x"] = population # HERE
  
  # set additional trace value
  Evolutionary.trace!(dt, objfun, state, population, method, options)
  Evolutionary.update!(tr,
      state,
      iteration,
      Evolutionary.value(state),
      dt,
      options.store_trace,
      options.show_trace,
      options.show_every,
      options.callback)
  end

deepcopy fixes the issue

This test fails with the old code, but passes with deepcopy:

# Test the the values of x are saved, not the reference
@test sol.original.trace[end].metadata["x"] !== sol.original.trace[end-1].metadata["x"]

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.01%. Comparing base (0a417ea) to head (d4c732a). Report is 23 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #730 +/- ## =========================================== + Coverage 11.96% 66.01% +54.05% =========================================== Files 21 22 +1 Lines 1438 1477 +39 =========================================== + Hits 172 975 +803 + Misses 1266 502 -764 ```

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