add other methods for taxonomy matching

[fdschneider]

if GBIF matching does not return a match, apply other nameservers.
Example:

get_gbif_taxonomy("Carabus arvensis")

[fdschneider]

The function standardize.taxonomy() takes an argument method which can be used to feed in any function that takes a table with a column 'scientificname' and returns a taxonomy lookup table (e.g. one that contains at least 'taxonID' or 'scientificNameStd').

Pull requests for other methods (most likely making use of the taxize package) are very welcome! Naming scheme should be get_xx_taxonomy().

[fdschneider]

in principle, providing own mapping functions is now possible by specifying 'method' and 'method_options' in function 'standardize_taxa()'.