Fix sum nodes initial colour during canonicalization
Fix sum nodes initial colour during canonicalization

In cases where the bounds could contain other dummies, the previous
initial colour could impose artificial ordering among summations.  Now
we only take the label as the colour.  This could solve this problem.
But better treatment of the bounds could also be possible.

tschijnmo/drudge,tschijnmo/drudge,tschijnmo/drudge