Refactored splitting of the scan operation in two phases

- sequential scan does not need to be split, so "perform" performs the
  whole simple scan algorithm, "performFirstPhase" only reduces the
  block (i.e. the whole vector), "performSecondPhase" performs the scan
  operation with the block result combined with a global offset as the
  initial value
- parallel OpenMP scan calls the sequential scan to process the block
  results
- parallel CUDA scan was changed such that the block results array is an
  exclusive scan after the first phase, same as in the other device
  specializations