fix examples in rankagg

[jangorecki]

There is an attempt to load vegan namespace in examples rankagg.Rd#L23, code below is the results from examples.
As vegan is suggested dependency, it should not be required to run examples according to R-exts. This can be conditionally escaped with requireNamespace.
It is possible there are other cases of that issue in other examples, this one was to first that fails.

* checking examples ... ERROR
Running examples in ‘taxize-Ex.R’ failed
The error most likely occurred in:

> base::assign(".ptime", proc.time(), pos = "CheckExEnv")
> ### Name: rankagg
> ### Title: Aggregate data by given taxonomic rank
> ### Aliases: rankagg
> 
> ### ** Examples
> 
> library("vegan")
Error in library("vegan") : there is no package called ‘vegan’
Execution halted

[sckott]

Thanks for this. Will fix