Practices when porting over variable names from papers

[genedan]

I ran into this issue when trying to do a Ruff fix on core/correlation.py:

# I is the number of development periods in the triangle
I = len(triangle.development)

The variable name needed to be changed because it was capitalized. Yet it can't just be "i" because it clashes with the indexing variable in the paper's various summations:

I've changed "I" to n_dev_periods, and changed the comment to say that this is the same as the capital I from the paper. This made me wonder whether we should have some guidelines in our docs on how to port over variables and formulae.

I'm thinking we should retain variable names when we can, but when we can't we should make sure to document in a comment which variable in the paper the new name refers to (A previous comment also refers to formula G4):

# n_dev_periods is the number of development periods in the triangle ("I" in the Mack 97).
n_dev_periods = len(triangle.development)