refactor `tax_glom_taxonomy_annotate()` to only have piped code block occur once by auto-inheriting `glom_var`

[taylorreiter]

In #37, I changed tax_glom_taxonomy_annotate() so that the user can select a glom_var. Right now you can only chose n_unique_kmers or f_unique_to_query. If I continue to expand the possible glom_vars, I'll refactor the code so that it uses the glom_var smartly and only has the piped code block once. It seemed like too much of a lift for something that might not even be that useful to implement this in #37.

[taylorreiter]

Also remember to update the function documentation to make it clear how the function works now.

[taylorreiter]

closed by #48