Uniquac more test

[SalvadorBrandolin]

Changes:

• UNIQUAC tests

More tests for the UNIQUAC model to discard a possible bug with temperature dependence in the Python API. There was
no bug, probably I had committed some dumb mistake when I was using it. More tests don't hurt anyone.

• UNIQUAC fixing

It has nothing to do with the temperature dependence "bug." I have changed the combinatorial term to allow the evaluation of UNIQUAC on a mole vector where some components have a composition of 0. Of course, all tests are working. The derivatives changes are reflected in the documentation too.

• UNIFAC change

I made the Ge_combinatorial and Ge_residual of UNIFAC methods of the type, which seems more correct if you want to evaluate one term only from a user perspective.

• UNIFAC docum

I moved all the noisy documentation (equations) of the Ge_combinatorial and Ge_residuals to the user documentation just like UNIQUAC documentation works.

• $\frac{dx_k}{dn_i}$ and $\frac{d^2x_k}{dn_idn_j}$ tests

The UNIQUAC changes don't use anymore those derivatives but I made two tests for those functions to preserve coverage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.99%. Comparing base (a98632d) to head (71d4536).
Report is 11 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #129      +/-   ##
==========================================
+ Coverage   90.75%   90.99%   +0.23%     
==========================================
  Files          73       74       +1     
  Lines        4034     4163     +129     
  Branches      306      308       +2     
==========================================
+ Hits         3661     3788     +127     
- Misses        299      302       +3     
+ Partials       74       73       -1     

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