Remove _gens from LinearCode

[Johan Rosenkilde]

It is stupid. And look through the uses of LinearCode.gens() if we shouldn't replace it directly with use of the generator matrix and take its rows.

[Johan Rosenkilde]

LinearCode.basis() is another function doing the same thing.

[Johan Rosenkilde]

There might be some Sage-space-convention reason for having either gens() or basis() or both. I mean, maybe by convention all spaces for which such a notion makes sense should support those two functions. We should check that out. But that still doesn't mean that self._gens needs to be cached.

[David Lucas]

Done.