Missing data analysis

[rosswilson-nz]

estimate() should work with multiple imputation data (from mice`)

• Dispatch on data argument
• Return a cea_mira class
• Pool method - should work with mice::pool() if appropriate glance() and tidy() methods are available (see #8)

[rosswilson-nz]

Added estimate.mids() and pool_cea()

pool_cea() is a custom implementation of mice::pool() that returns a mipo-like object (with class cea_pooled/cea_estimate)

We need this custom implementation because the full vcov matrix is needed for bootstrapping and calculation of CIs (for e.g. INMB) and CEAC -- mice::pool() only computes the estimate & SE/var seperately for each parameter