Backports for 2.x

[palday]

• Grouping for 2.x (Grouping for 2.x #353)
• use weights when fitting LMM (use weights when fitting LMM #334) (Backport reweighting bug fix #357)
• fix propogation of defaults in non-column table contructor (fix propogation of defaults in non-column table contructor #398)
• check GLMM stderr (correct coeftable for GLMMs, add tests #308)
• document problems with GLMM families with dispersion parameter (warning for GLMM with dispersion parameter #373)
• change GLM column labels (change CoefTable column names to match GLM.jl #335)
• `ranef method for GLMM (ranef method for GLMM #336)
• make (1|x) + (y|x) equivalent to (1|x) + (0+y|x) (make (1|x) + (y|x) equivalent to (1|x) + (0+y|x) #338) (too much effort, potentially can be interpreted as breaking, and 3.0 is coming out soon)
• use wts argument in GLMM constructor when conversion to column table is necessary (fix propogation of defaults in non-column table contructor #397, fix propogation of defaults in non-column table contructor #398)
• fix ranef docstring (change ranef doc string to match default args #355)

(Since we depend on GLM for other things, maybe it's possible to extract these automatically from GLM? Perhaps we need an upstream change to have their col names accessible as a constant?)