IMP: Standarize naming convention and function signatures

[fedebenelli]

In some cases derivatives are defined like:

• $\frac{dG^E}{dn_i} = $ Gen
• $\frac{dD}{dn_i} = $ dDi
• $\frac{d^2G^E}{dTn_i} = $ GeTn
• $\frac{d^2D}{dTn_i} = $ dDiT

Some standardization for this would be nice. Which should also be added to STYLE_GUIDE.md

This also extends to the use of self or eos or model, and order of variables f(eos, v, T) vs f(eos, T, v) for example

[SalvadorBrandolin]

What do we do with the upper and lower case of variables?

1. n V T P
2. n v t p

[fedebenelli]

Being domain-specific variables I would keep them as they are usually viewed in equations. There si the little detail that volume probably should be v instead of V when we talk about molar volume