Add tp_flash & example notebook

[denbigh]

This pull request contains a new file, src/methods/property_solvers/tp_flash/DifferentialEvolutiontp_flash.jl, in which a new function, tp_flash, is defined and exported. tp_flash solves the non-reactive flash problem via differential evolution.

I have also included an example notebook, examples/PTFlashDifferentialEvolution.ipynb, to demonstrate use of this function.

I am new to package development in julia, so I apologize if changes need to be made.

Feel free to free to use and modify this code in any way.

[pw0908]

Thinking in terms of tests, what would say is the minimum required to ensure all aspects of your code works?

Would just testing for two-phase equilibrium be sufficient? Or should we test three-phase as well?

[denbigh]

I think a two-phase test would be sufficient. There are a number of examples (with 2 and 3 species) in the example notebook. A three-phase simulation should work as well, but I believe you'd need >=4 species to see a 3-phase region of phase space at fixed p and T.

[pw0908]

There's a compatibility issue between BlackBoxOptim.jl and FillArrays.jl which we're trying to resolve. Really, we're just waiting for the BlackBoxOptim.jl folks to update their package...

[pw0908]

I've now switched the optimiser package to Metaheuristic such that we don't have a compatibility issue. I've also added a simple test. Do you think that's sufficient?

[longemen3000]

If the test passes, merge it, we can iterate over it later

[pw0908]

Do you want to re-organise anything?

[longemen3000]

Oh, yes, but if the functionality is working, the code reorganization can be done later

[codecov-commenter]

Codecov Report

Merging #56 (fb57f90) into master (92a613d) will increase coverage by 0.01%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   84.35%   84.37%   +0.01%     
==========================================
  Files         112      113       +1     
  Lines        6877     6917      +40     
==========================================
+ Hits         5801     5836      +35     
- Misses       1076     1081       +5     

Impacted Files	Coverage Δ
src/Clapeyron.jl	100.00% <ø> (ø)
..._solvers/tp_flash/DifferentialEvolutiontp_flash.jl	87.17% <87.17%> (ø)
src/solvers/nlsolve.jl	66.66% <0.00%> (+1.96%)	⬆️

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[pw0908]

Alright, after 3 runs, I still can't get all tests to pass but I think Andrés' other update should be able to handle it.

[denbigh]

Wonderful, thanks for making this process so smooth! I can see you have made my crude code more 'Julian' - I look forward to testing this to see if there are some performance improvements!