[Rd] Overwrite S3 methond from base package

[Thaler,Thorn,LAUSANNE,Applied Mathematics]

> In regard to "should", I think it is a
> very very bad idea.
> 
> The result that someone gets depends on
> whether or not the new package is attached
> in the session.  That is a recipe for hours
> (or days) of trying to figure out mysterious
> behavior.

I fully agree. However, there are 2 reasons in this particular case, why
I decided nevertheless to be "evil":

1. The package will never (well, never say never, but from today's point
of view I see no reason, why this should ever happen) be published and
is intended for internal use only. 
2. Even more important: I think that 'droplevels' in the base package
has a major flaw as it silently drops any contrast chosen by the user.
Hence, a user who wants to use droplevels, has to reassign all contrasts
afterwards. In another discussion it was pointed out, that contrasts
which worked fine for a factor, may be erroneous once a level is dropped
(in the case we specify the contrasts matrix directly). I acknowledge
this case (and so does my function as it falls back to the base version
but issues a warning), yet I think for all the other cases, where the
contrast is defined via a function, it would be the better behaviour to
use the same contrast function rather than dropping it silently. As a
consequence, I think that my function does no harm, for it does exactly
the same as the base function while adding more functionality. Thus,
even if the user is used to the annoying behaviour of base::droplevels
and reassigns the contrast after each use of droplevels, he will see no
difference whether or not my package is loaded. In the former case the
step is however not necessary.

For the "how" part, I find out that I have to explicitly export my
functions such that they can override the ones from the base package.
Thus the solution was to add

export(droplevels.factor, droplevels.data.frame)

to my NAMESPACE file.

Thanks for your input,

-Thorn