equal scaling in ggplot ordination figures

All ordinations where the Euclidean distance on the plot reflects/approximates some underling dissimilarity in full dimensions require the axes to be equally scaled. In **ggplot** this is achieved via `coord_fixed()`. This patch implements this.

Also note that this could be done with

    require(ggvegan) ## install from [github](https://github.com/gavinsimpson/ggvegan)
    autoplot(ord)

But that might be too large a change to the spirit of the slides.