Migrate GEE engine support to glmtoolbox::glmgee()

[mlotinga]

Feature

It would great if the current support for Generalized Estimating Equations solving for GLMs via engine 'gee' could be migrated to the much richer modelling available in the form of glmgee() and associated methods and functions in the glmtoolbox package https://cran.r-project.org/web/packages/glmtoolbox/index.html . Other than the richness of the package, another advantage is the similarity with glm, which makes support incorporation easier - for example see discussion relating to emmeans, which has started to incorporate some support : rvlenth/emmeans#454