Standardize on terminology: r(X) vs I(x)

[kevaundray]

This has been taken from Benedikt Wagner's document.

Link:

rust-eth-kzg/cryptography/kzg_multi_open/src/naive.rs

Line 104 in e043452

let r_x = lagrange_interpolate(&coordinates).expect("lagrange interpolation failed");

Creating the issue as this is done in a few places in that file -- I think the comments should use I(x) and just clarify that this is the "remainder polynomial".

Related to #246