Add dtype and flags to CML

This PR is essentially a null change. It just records byte order and array order information in the CML files.

The intention is to establish a known baseline before considering the byte/array order implications of introducing biggus.

NB. The DataManager class forces a native byte order when loading data. The data checksums in the CML files are computed over these native byte order arrays. As a consequence, the *existing* result files are architecture-specific. This PR makes no attempt to address that issue.