SymmetricGroupAlgebra: updates w.r.t. categories and free modules

[nthiery]

The goal of the patch is to adapt SymmetricGroupAlgebra? to categories and to add some improvements. The patch improve SymmetricGroupAlgebra? in two ways:

• SymmetricGroupAlgebra? is now in the category FiniteDimensionalAlgebraWithBasis?. Note: A forthcomming patch from Valentin Feray will put it in the correct GroupAlgebras? category;

• When creating SGA(n) a coercion from SGA(n-1) is declared.

I'm submitting the patch on behalf on Nicolas after reviewing it.

Depends on the categories framework #5891, #6136.

Cheers,

Florent

[mhansen]

For this, isn't it possible to lazily add the coercion using coerce_map_from?

[nthiery]

Replying to mhansen:

For this, isn't it possible to lazily add the coercion using coerce_map_from?

Probably so. It would be best handled by some "templated coercion declarations", as I had started to implement in MuPAD. Let's just leave it as is for the moment, until we have enough use cases to come up with the right design.

[nthiery]

Updated patch fixes two missing doctests (apply only this one)