[ENHANCEMENT] Better handling of dual

[ogauthe]

To produce a singlet, the domain and codomain must fuse to dual sectors. Currently, both domain and codomain are fused on the same sector. The flip on the domain should be explicit.

[ogauthe]

I pushed a commit where I explicitly flip the domain fusion tree. I am not entirely satisfied with it, but internal consistency imposes several constraints:

• the data_matrix has to be diagonal (being off diagonal is very confusing)
• it imposes a given order for sectors
• I do not want to have completely different code paths for domain and codomain

I am not settled yet whether I prefer the new version or the old one, I am not sure it is much more clear.