new_taxon_rank: Minimal taxon_rank constructor
In zachary-foster/taxa2: Classes for Storing and Manipulating Taxonomic Data

Minimal taxon_rank constructor for internal use. Only use when the input is known to be valid since few validity checks are done.

1	new_taxon_rank(rank = character(), levels = taxon_rank_level())

`rank`	Zero or more taxonomic rank names. Inputs will be transformed to a `character` vector.
`levels`	A named numeric vector indicating the names and orders of possible taxonomic ranks. Higher numbers indicate for fine-scale groupings. Ranks of unknown order can be indicated with `NA` instead of a number.