Rename prefix sum methods in vectors to scan

The former PrefixSum class was renamed to Scan, but the methods in vector classes (Vector, VectorView, DistributedVector and DistributedVectorView) stayed as prefixSum and segmentedPrefixSum - see e.g. https://mmg-gitlab.fjfi.cvut.cz/gitlab/tnl/tnl-dev/blob/develop/src/TNL/Containers/Vector.h#L269

While we're renaming things, exclusiveScan method should be added as a shortcut for vector.template scan< TNL::Containers::Algorithms::ScanType::Exclusive >(). Both methods should be used in the tutorial.