`search_taxonomy` fails to search 'below' informal nodes

[mjwestgate]

In version 1.3.0, the following code fails to detect a lot of taxa, apparently because it stops searching at nodes labeled informal or unranked. This leads to some very visible problems, e.g.

search_taxonomy("Animalia", down_to = "class")

....fails to return any of the vertebrate classes, because they are all listed under the informal rank Gnathostomata. This appears to occur because the recursive function inside search_taxonomy (level_down()) checks against find_ranks()before proceeding, and informal is not listed in find_ranks. Suggest allowing level_down to recognise informal nodes.

[mjwestgate]

This has been addressed with the following steps:

• allow level_down() to skip past ranks labelled informal or unranked
• swap level_down from using data.frame to list, enabling data on intermediate nodes to be stored
• convert resulting object from list to tree using data.tree to allow extraction of richer node-level data
• swap function name from search_taxonomy to ala_taxonomy for consistency with galah naming conventions for data-returning functions
• force ala_taxonomy to require object of class ala_id (via select_taxa()) for consistency with with other ala_ functions