Template:XC3 class stat mod

Takes as arguments a class rank (from 1 to 20) and two values representing a stat mod at class rank 1 and rank 20. Calculates the rest of the values for the stat mod between the two ranks.

Assumptions:
 * If rank20 = rank01 + 10, then it goes up every 2 levels (1 to start with).
 * If rank20 = rank01 + 5, then it goes up every 4 levels (2 to start with).
 * If rank20 = rank01, then it's the same at all ranks.
 * There are no other cases.

Usage:

returns the value at rank 5 of a stat mod that's 80 at rank 1 and 90 at rank 20.