Implement functions in SequentialInformation

Summary

Implement dot() and norm() in Amg::SequentialInformation to have it closer to a sequential analog of parallel information like OwnerOverlapCopyCommunication.

Discussion

Why should dot and norm not be called in a sequential setup? Maybe this SequentialInformation is mostly used for tagging functions. If so, then a shorter tag::sequential would be cleaner. If the interface is required but is not allowed to be used in a sequential setup then this is a design flaw. But, maybe I just don't understand the usage of this class. In a generic setting it would be nice to have a sequential analog to the parallel communicators, though.