Ordination plots should use equally scaled axes

[gavinsimpson]

This PR fixes the problem with ordination diagrams raise in Issue #176 through the use of ggplot2::coord_fixed() to make the axes of the plot use equal scaling.

[KasperSkytte]

I'd rather make this optional with a default, since it wouldn't be easily turned off by the user.

[gavinsimpson]

Then you misunderstand the problem; these are incorrect plots if you don't use a plot with the same scale on both axes, the biplot interpretations are messed up. I don't think we should give users the option to create incorrect plots.

[gavinsimpson]

I may not have fully understood all the options in your code and add the equal scaling to all plots - if so, that wasn't intentional and the correct fix is to apply this to those plots when two ordination axes are being plotted.