[ENHANCEMENT] refactor `clebsch_gordan_tensor`

[ogauthe]

Currently, clebsch_gordan_tensor is defined in FusionTensor. We may move it to SymmetrySectors. We may also define another interface with symbol_3j.

[ogauthe]

There are 3 points here:

• where to define clebsch_gordan_tensor: either in SymmetrySectors or here
• how much do we want to depend on TensorKitSectors: we could call it instead of defining CG coeff ourselves
• decide whether we prefer symbol_1/3/6j or flipper, clebsch_gordan and f_symbol interface.

[lkdvos]

I think it could be very convenient to try and recycle as much of TensorKitSectors as possible: this is all mostly data, so there are not too many considerations about different implementations. There is already quite extensive support for different symmetries, and it would be great if we could keep this centralized: if anyone needs a particular group, this would only have to be defined once. Additionally, I separated this out specifically to improve reusability for ITensors, if there are glaring missing or inconvenient parts about that, we can definitely also discuss if there are changes to TensorKitSectors that could improve the overall experience.

[mtfishman]

Calling out to TensorKitSectors makes sense to me.