Improved initial point for saturation pressure

[longemen3000]

at the cost of one additional gas pressure evaluation (but probably compensated by less evaluations in the liquid branch), x0_sat_pure now interpolates a vdW EoS between a liquid point and a gas point near the spinodal (before, it did the same,but using the second virial coefficient directly). Using a correlation, we can now (mostly, but couldn't found cases were this didn't hold) evade the old branch where the liquid volume couldn't be calculated due to the predicted pressure being too low (happening specially near the critical point).
Furthermore, this now includes a dual spinodal solver that allows better initial points in 0.97 < Tr < 0.995, without the need of calculating the critical point, while still being faster than a crit_pure calculation.
this improves dramatically the calculation speed of eos with long chain components. the main focus was to calculate properties of model = PCSAFT("eicosane") near the critical point without the need to actually calculate it.

It also adds some utility functions, mainly an quintic hermite interpolator, used for the spinodal solver

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (8bf867e) 79.56% compared to head (2e37d82) 79.94%.
Report is 9 commits behind head on master.

Files	Patch %	Lines
src/methods/initial_guess.jl	91.08%	9 Missing ⚠️
...rty_solvers/singlecomponent/saturation/ChemPotV.jl	86.66%	4 Missing ⚠️
src/modules/solvers/poly.jl	83.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   79.56%   79.94%   +0.37%     
==========================================
  Files         280      279       -1     
  Lines       19421    19484      +63     
==========================================
+ Hits        15453    15577     +124     
+ Misses       3968     3907      -61     

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