cran comments for submission of v0.3.0

export a '\value' block for dupree() (#47)

An earlier commit had a mistake, where an '<AT>value' tag was used in
the roxygen2 for dupree(); this should have been an '<AT>return' tag.

Also
- increased the consistency of the documentation ('code-block' should be
used in the docs, not 'code block')
- exported the \seealso tags for `dupree_dir()` and `dupree_package()`