Moved implementations of scan and distributed scan into the detail namespace

The algorithms are supposed to be used via overloaded plain functions in
the Algorithms namespace: for now, there are only inplaceInclusiveScan
and inplaceExclusiveScan (and their distributed variant).

The scan and segmentedScan methods were removed from data structures
(Vector, VectorView, DistributedVector, DistributedVectorView). They
were inflexible (only std::plus was actually used for reduction),
incomplete (some overloads just threw NotImplementedError), and they
were violating the open-closed principle:
https://en.wikipedia.org/wiki/Open%E2%80%93closed_principle