Include math in conjunction with functions

[ivelasq]

From the Spring Seminar:

• [Workshop 1 survey] I'm sure this would be helpful for some and not for others, but I think I would have found it helpful to map a little more explicitly what the math was that was being done in each step. What exactly is being created when we create a design object? What are the default settings and what are some examples of what we might set them to instead? For me, I think part of understanding this clearly is to see the mathematical setup for the models/methods we are about to implement through code, then see the code for them (or flip back and forth).
• [Workshop 2 notes] Had a good question about what happens when there are missing values in numerator or denominator for svyratio. Does missing values in one variable affect the total for the other variable? Answer is yes.