[Docs] State conventions for various `Sector` implementations

[lkdvos]

For each Sector type, we should try and at least document:

• The labels of each sector
• The basis choice of fusiontensor

One way of specifying the basis choice might be to include matrices for the generators of a group in the various representations.
This should uniquely determine the basis, be somewhat human-readable, and provide a way of "testing" the conventions.