Clarify coding of factors in example datasets

[mjskay]

Currently datasets like Higgins1990Table5 use codings like "1", "2", "3", ... for factors, which can look like numeric data in examples and printed tables even though they aren't. I suspect this contributes to people passing in numeric data for factors (see issue #5).

Simple solution here is to rename the levels to things that are obviously strings and not numeric.