In ropenscilabs/taxa: Classes for Storing and Manipulating Taxonomic Data

Adding getters and setters

Make consistnet getters/setters for all classes with the following characteristics:

The source values are private and prefixed with my_
The active bindings return objects, but check the object type when setting and can accept vectors when setting. They basically pretend to be the source values.
objects returned by active bindings can be converted to characters by as.character
Remove the ability to store vectors instead of objects. Vectors given to active bindings are converted to objects automatically.
Use taxon_names, taxon_ranks, and taxon_ids S3 functions for all classes as vector-based getter/setters. Make them usable as setters( e.g. taxon_names<-). It might be a bit odd to use taxon_names to get/set of taxon objects, but having only one function name for everything will make it easy to use.
Make taxonomy/taxmap and hierarcy inherit taxa, so its getters/setters work for them.
S3 setters pass by value, R6 setters pass by reference
All vailidity checks are in the active binding setters when possible. Any ones that rely on combinations of arguments are in the class initialization after the active binding checks