Support emmeans

[rdinnager]

Support the emmeans package for estimating marginal means from communityPGLMM objects. There is a vignette for developers here:
https://cran.r-project.org/web/packages/emmeans/vignettes/xtending.html

On first glance this shouldn't be too difficult?

[RobJamesRamos]

Second this request. It should be possible to work around the lack of support by using the qdrg function in emeans. I have tried to get it to work just providing the formula, original data, coefficients, and variance-covariance matrix. I think I figured out how to get the coefficients out of the model object mod_pglmm$B[, 1]. I am not sure how to get the variance-covariance matrix out. Cant use vcv as it does not have a method for class emeans.