CPA revamp

[longemen3000]

• unifies sCPA and CPA by using just a symbol to select rdf
• allows Pc to be optional, so you only pass (a,b,c1,Tc) for non-assoc and (n_H,n_e,epsilon_assoc,bondvol) as additional parameters for assoc
• looking at the literature, there is a split on passing epsilon_assoc as energy parameters or as temperature parameters (ϵ_ab/R). we now support both, via passing epsilon_assoc_over_R.
  addresses CPA example please #239, solves Move CPA input to use SI values #242

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (af65700) 79.61% compared to head (8f3bb4a) 79.57%.

Files	Patch %	Lines
src/models/SAFT/CPA/CPA.jl	80.48%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   79.61%   79.57%   -0.04%     
==========================================
  Files         280      280              
  Lines       19397    19384      -13     
==========================================
- Hits        15443    15425      -18     
- Misses       3954     3959       +5     

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