CMakeLists.txt

- Define ItemValue
  allow to define generic arrays defined on connectivity items
  data is accessible through item identifier (avoids confusion e.g. calling cell
  data using a node index). Cannot use integer indices to access data.

- SubItemValuePerItem is now indexed by item identifier

- Introduce Array<T> which encapsulates Kokkos::View<T*>
  Array replace Kokkos::View when ItemValue cannot be used (not data per item)
  ItemValue internally uses Array

- ReduceMin and ReduceMax are defined as generic class-functions operating on
  arrays (which must honor .size() and ::data_type).
  Casts to the type of the content of the array to obtained reduced value.

- Encapsulate as many Kokkos feature as possible

- Code clean-up
  'typedef' is replaced by the cleaner 'using' type aliasing