Code editing

Several fixes in the PR, which hopefully simplify the R code used in chunks.

 1. `top.pinus` was returning from two locations. This is not good (in general) form so I simplified this to a single (implicit) return
 2. use `subset()` for its intended purpose. Simplifies the subsetting for sample age and longitude
 3. Pandoc will sensibly render `--` as an en dash, the correct typographical mark to separate two numbers.
 4. use `transform()` to simplify the coercion to factor
 5. minor indentation fix