Define one style of math notation

[erikhuizinga]

The articles by Bland and Altman use some mathematical notations that differ from other publications or some modern conventions. This GitHub entry should incorporate a single mathematical style, clearly defined throughout the functions and in the accompanying mathematical descriptions, e.g. in the readme, a future wiki or other documentation of the mathematics.

For example, the difference between measurement methods be written as d = X - Y or D = X - Y. The code should reflect the mathematics. Variables for programmatic purposes instead of mathematical should be clearly different from the mathematically meaningful. For example, the X vs. x variables from here have a programmatic use, but the notation could be confusing. It would be better to name them more clearly, so they can be more easily understood.