Missing `Sum` classes in syntax modules

[y-richie-y]

Sum classes are missing in the following syntax modules:

• traced
• pivotal
• ribbon
• compact
• frobenius

from discopy import *

f = traced.Box('f', markov.Ty('a'), markov.Ty('a'))
dub1 = traced.Functor(ob=lambda x: x, ar=lambda x: x >> x)
dub2 = traced.Functor(ob=lambda x: x, ar=lambda x: x + x)

dub1(f) # works
dub2(f) # fails

[toumix]

There were missing Sum classes in some of the modules. I've added the ones up to markov but I'll rename this issue to deal with the rest.