Fix units / res args for *png* plotting device.

Update the `units` argument so that it accepts values from ["px", "in",
"cm", "mm"].

The `res` argument was added for the cases where `units` is specified
and is not default (px). A default value of 72 is provided by rmagic in
the case that `units` is not "px" but `res` is not specified.
Add units option to Rmagic test
Merge branch 'fix-rmagic'