Support for geeglm

This version adds support for geeglm() models from geepack. It also adds support for meta-analytic location-scale models estimated using the metafor package, along with miscellaneous bug fixes and enhancements.

• Added support for geepack::geeglm() models.
• Added support for rma.ls models (location-scale models estimated using metafor::rma.uni(scale = )).
• Improved error handling of Wald_test() when vcov of contrasts is not positive definite.
• Fixed a bug in linear_contrast() to handle specified contrasts that are scalars.
• Improved internal get_data function for gls and lme objects to allow use of expressions in addition to object names.