Sources example files
Facilitates learning a new package, by sourcing any specified example file for an installed package.
Package name, quoted.
Desired example file name, if any, quoted.
Subdirectory name of the examples directory in the package.
Loads the example file
exfile, from the
directory within the tree where
pkg is installed.
exfile = NA, the names of the files are
returned, without any loading.
See the case
exfile = NA above.