Return information about external file
[name,offset,size] = H5P.get_external(plist_id,idx)
[name,offset,size] = H5P.get_external(plist_id,idx) returns
information about the external file specified by the dataset creation
property list plist_id. The idx specifies
the external file index, which is a number from zero to N-1,
where N is the value returned by H5P.get_external_count.
The name output returns the name of the external
file (limited by 2048 characters). The offset output
returns the location in bytes, from the beginning of the external
file, where the data starts. The size output returns
the size of the external data.