bug fix for maximum numeric value being plotted

Hi Martin,

I noticed a bug in which the maximum value in a numeric matrix was not being plotted.
This was because of the line (fixed here) in `prep.numeric` which coerced the numeric values to character and back again; this caused a rounding error and meant the highest value was above the highest value in `plot.control$line.breaks`, so the assigned colour was NA and no line plotted for the strongest link.

There's an MRE of the bug in this gist: https://gist.github.com/goldingn/bcbf066717fb0618ae3c
you can switch between the two versions at the top of the gist.

Thanks for all your work on this package, it produces some really nice results!

Cheers,
Nick