[Rivet] Updated Cuts system prototype/demo

[David Grellscheid]

Hi Andy,

I like it, it's really neat now! I have the feeling that something can 
still be condensed a bit more, but I'll have to play with it. The 
Softsusy code successfully hides the template stuff completely away from 
the end users, so I must have had a way of achieving that 10 years ago 
without C++11 :-D

I'll copy the pastebin and send you a diff if I get it sorted.

See you,

   David