Improve syntax of margin input with lists

The idea is to change the recommended margin input method to something like:

marginList <- list(
"categ" = c(0.35, 0.4, 0.25),
"sexe" = c(0.60, 0.40),
"service" = c(0.45, 0.55),
"salaire" = 470000
)

- Add tests with the list method, with categorical and continuous variables

- Modify addMargin

- marginMatrix becomes an ambiguous parameter name: consider changing it (but ensure retrocompatibility!)

- Change recommended way of integrating margins in the docs

Thanks to Toby Hocking for the suggestion!