New function pool.table() to allow for non-R workflows

[stefvanbuuren]

This PR promotes a system-independent method for pooling.

The PR introduces a new function pool.table() that takes a tidy table of parameter estimates stemming from m repeated analyses. The input data must consist of three columns (parameter name, estimate, standard error) and a specification of the degrees of freedom of the model fitted to the complete data. The pool.table() function outputs 14 pooled statistics in a tidy form.

The primary use of pool.table() is to support parameter pooling for techiques that have no tidy() or glance() methods, either within R or outside R. The pool.table() function also allows for a novel workflows that 1) break apart the traditional pool() function into a data-wrangling part and a parameters-reducing part, and 2) does not necessarily depend on classed R objects.

Related #95, #105, #106, #190, #207.

[hanneoberman]

Added new pool_table() function to the pkgdown website reference to solve check error (commit b927614). Then further updated the website docs to fix site images (browser tab icon and logo size, see problems below)