Made the standalone version of AlgSequence a bit more flexible.

Taught it how to accept insert requests, and how to delete an existing
algorithm/sequence from the sequence.

Did not introduce a __delitem__ function, since Athena configurables
don't have such a function defined on them either.


Former-commit-id: 6f8fbf4af3f4c3904d541e2d06c37ebc2dd9f9c3