Issue #9
Added new header field <manifest> for an abbreviated, human-readable
summary of the contents of a dataset: this will be copied into the
"extent or quantity" field of new ReDBox records.
Added a manifest method to the Dataset object.
Added methods size and format to the Datastream object - these are used by
the Dataset::manifest method.
Added cursory checking in 004_metadata_xml.t to see that the manifest element
is being filled with some non-null content.