Various edits to the Intro to RNeXML section 

Includes an additional reference (to TreeBASE).
Yup, the example demonstrates this.  `comp_analysis.xml` includes two traits, one of which is a `nex:ContinuousCells` block and one of which is a `nex:StandardCells` block, and the `get_characters()` example here shows them combined into a single `data.frame`.