[Rivet] Weight menace

[David Grellscheid]

Hi all,

> The whole nominals central weight discussion
> could be circumvented if we
>
>   * check on initialisation if there is more than one weight in the
>     first place
>   * treat all weights the same, i.e. always write out PATH[weightname]
>   * tell the plotter what the central weight is --- if there is more than 1

Yes please! That's the only sensible choice that's reasonably 
future-proofed.

   David