Base graphics support

[zeileis]

Steve, thanks again for putting together this package. As suggested on Twitter (https://twitter.com/AchimZeileis/status/1459103077798191130) I have added a painbow.colors(n, alpha = FALSE, rev = FALSE) function that will be particularly useful for base graphics.

In addition to the function itself, I made the following modifications:

• Adding the corresponding documentation to the main painbow.Rd file, along with base graphics examples.
• Adding the necessary base packages (grDevices, graphics, stats) in the DESCRIPTION along with some other changes and an increase of the minor version to reflect the new functionality.
• Explicitly importing/exporting the functionality in the NAMESPACE.

I hope this facilitates integrating the modifications into the package. Happy to make further improvements if necessary.

Best wishes, Achim