Rework reductions

- define min, max and sum functions for generic array types. These are
  local (i.e. non-parallel) reductions
- define min, max and sum functions for item value types. These are
  parallel reductions

Parallel versions are not finished since
- ghost items need to be ignored
- mpi sum reduction needs to take into account non scalar
types (i.e. TinyVector's and TinyMatrix's for instance)