Explore adding multivariable analyse_pars() and pool_pars() functions

[wolbersm]

At the moment, analyse() and pool() just look at univariate estimates (plus se, df) and pool them one-by-one.
However, for several analyses (e.g. subgroup analyses, multi-arm trials with Dunnett's adjustment, Fieller's CI for ratios etc), it would be useful to be able to have two functions:

• analyse_pars() where the output from each imputed dataset is a vector of estimates (plus corresponding covariance matrix)
• pool_pars() which pools estimates (plus covariance matrices) across imputed datasets

[gowerc]

My personal preference would be to integrate this functionality into the existing analyse/pool functions as opposed to creating a duplicate pair. Perhaps as either a flag option or method dispatch based on the input. This is something we can look into / consider though.