Consider adding function(s) to do standardisation

[simonthelwall]

Problem statement

Ken Rothman's episheet has a tab for calculating standardised rates and proportions. This package has no such functions.

Proposed solution

Add one or more functions to calculate standardised rates and proportions.
See Ch. 15 from p265 of 2nd Ed Modern Epidemiology.
It should fit with tidyverse so that can do grouped calculations.
Table 15-2, p268 has sample data (which I think already exists in package?), same as on standardisation tab in Excel episheet.