The item location box provides a directory of resources in this or other
files, by locating their containing file, their offset within that file, and
their length. Placing this in binary format enables common handling of this
data, even by systems which do not understand the particular metadata system
(handler) used. For example, a system might integrate all the externally
referenced metadata resources into one file, re-adjusting file offsets and
file references accordingly.

Items may be stored fragmented into extents, e.g. to enable interleaving. An
extent is a contiguous subset of the bytes of the resource; the resource is
formed by concatenating the extents. If only one extent is used then either
or both of the offset and length may be implied:

• If the offset is not identified (the field has a length of zero), then
  the beginning of the file (offset 0) is implied.


• If the length is not specified, or specified as zero, then the entire
  file length is implied. References into the same file as this metadata, or
  items divided into more than one extent, should have an explicit offset and
  length, or use a MIME type requiring a different interpretation of the file,
  to avoid infinite recursion.


The size of the item is the sum of the extent lengths.

The data-reference index may take the value 0, indicating a reference into
the same file as this metadata, or an index into the data-reference table.

Some referenced data may itself use offset/length techniques to address
resources within it (e.g. an MP4 file might be 'included' in this way).
Normally such offsets are relative to the beginning of the containing file.
The field 'base offset' provides an additional offset for offset calculations
within that contained data. For example, if an MP4 file is included within a
file formatted to this specification, then normally data-offsets within that
MP4 section are relative to the beginning of file; the base offset adds to
those offsets.

@author Alexander Simm

The item ID is an arbitrary integer 'name' for this resource which can be
used to refer to it (e.g. in a URL).

@return the item ID

The data reference index is either zero ('this file') or a 1-based index
into the data references in the data information box.

@return the data reference index

The base offset provides a base value for offset calculations within the
referenced data.
return  the base offsets for all items

The extent offset provides the absolute offset in bytes from the
beginning of the containing file, of this item.
return  the offsets for all extents in all items

The extends length provides the absolute length in bytes of this metadata
item. If the value is 0, then length of the item is the length of the
entire referenced file.
return  the lengths for all extends in all items