Add examples for partitioning constrained designs

[leeper]

Please specify whether your issue is about:

• a possible bug
• a question about package functionality
• a suggested code or documentation change, improvement to the code, or feature request

When designs are constrained, AMCEs for the constrained features are only defined over subsets of the other features against which they are constrained. It would be nice to have an example (e.g., using the immigration dataset) that shows how to clearly partition based on the constraints.

[leeper]

For example, note how MMs are undefined for Job categories when Education is subset:

data("immigration")
f1 <- ChosenImmigrant ~ Gender + Education + Job + LanguageSkills + CountryOfOrigin + ReasonForApplication + JobExperience + JobPlans + PriorEntry
cj(subset(immigration, Education %in% c("No Formal", "4th Grade", "8th Grade", "High School")), f1, id = ~ CaseID, estimate = "mm")

and vice versa:

cj(subset(immigration, Job %in% c("Financial Analyst", "Computer Programmer", "Research Scientist", "Doctor")), f1, id = ~ CaseID, estimate = "mm")

We should also add equivalent AMCE examples and plotting approaches.