deprecated functions: scatter_classic, quiver_classic, colorbar_classic

In June I asked if anyone was still using pcolor_classic

    > (
    > I got no replies, so I have now removed it from svn.

    > Next: it looks like scatter_classic dates from the same era.
    > Does anyone have any objections to its removal?

The only suggestion I have is rather than simply removing them, we
might go through a release cycle with something like

    def pcolor_classic(self, *args, **kwargs):
        raise NotImplementedError('pcolor_class is deprecated, please use pcolor instead')

so people will not have to post questions saying 'what happened to