Store constant AMR arrays with less memory

The AmrArrays filter generates some cell fields that specify information about the hierarchy, which are constant across all cells in a partitions. These were previously stored as an array with the same value throughout. Now, the field is stored as an ArrayHandleConstant, which does not require any real storage. Recent changes to VTK-m allow code to extract the array as a component efficiently without knowing the storage type.

Fixes #794 (closed)