Assoc options

[longemen3000]

fixes #54
to each site EoS model, there is now a new keyword, assoc_options containing an AssocOptions struct of the form:

assoc_options = AssocOptions(atol = 1e-12,rtol = 1e-12,max_iters = 1000, dampingfactor = 0.5)

if one need to increase the max iterations of an specific model, now you could just do:

model = PCSAFT(components,assoc_options = AssocOptions(max_iters = 10000))

as a byproduct, split_model was refactored to be a little bit more general. the only special case we do now is the references field.

Also, there is take 3 of trying to improve NaN handling. the problem was that ForwardDiff.Duals wasn't being processed by our custom log (and now sqrt). with the new definitions, NaNMath is no longer used, and can be safely removed in a subsequent commit.
It seems that our previous saturation pressure solver wasn't robust enough, as now there aren't any errors in the test suite with the default log implementation.

i will merge after #56

[codecov-commenter]

Codecov Report

Merging #57 (c4ff7d3) into master (8d105fb) will increase coverage by 0.05%.
The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   84.35%   84.41%   +0.05%     
==========================================
  Files         112      112              
  Lines        6878     6871       -7     
==========================================
- Hits         5802     5800       -2     
+ Misses       1076     1071       -5     

Impacted Files	Coverage Δ
src/Clapeyron.jl	100.00% <ø> (ø)
src/utils/split_model.jl	81.73% <87.50%> (+2.42%)	⬆️
src/solvers/Solvers.jl	87.27% <88.88%> (+6.91%)	⬆️
src/models/eos/SAFT/BACKSAFT/BACKSAFT.jl	100.00% <100.00%> (ø)
src/models/eos/SAFT/CKSAFT/CKSAFT.jl	100.00% <100.00%> (ø)
src/models/eos/SAFT/CKSAFT/variants/sCKSAFT.jl	100.00% <100.00%> (ø)
src/models/eos/SAFT/CPA/CPA.jl	100.00% <100.00%> (ø)
src/models/eos/SAFT/LJSAFT/LJSAFT.jl	100.00% <100.00%> (ø)
src/models/eos/SAFT/PCSAFT/PCSAFT.jl	85.50% <100.00%> (ø)
src/models/eos/SAFT/PCSAFT/variants/sPCSAFT.jl	90.32% <100.00%> (ø)
... and 11 more

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