function ncbi_children

This adds the ability to search for children taxa using ncbi names or uids by introducing the functions `ncbi_children` and `ncbi_get_taxon_summary`. When searching by name, different taxa with the same name (e.g. Satyrium) can be differentiated by supplying a character vector of ancestor names (See examples in roxygen comments). I have also added a method for class `uid` for the children generic and updated the documentation.