print citations to a file

[LunaSare]

Created function get_citations(), to obtain citations from supporting studies for a subtree or induced subtree. Added argument file_citations to function tol_induced_subtree(). It accepts a file name for citations to be written to. Created documentation with roxygen2::roxygenise(). We ran the checks locally and it works. If you approve, we can add this functionality to tol_subtree() function too.