Extension to XDF to represent FITS HDUs - version 0.04 SIMPLE is not used. * NAXIS is not used, the API counts them. * BITPIX info is held in XDF:array/dataFormat. * BUNIT info is held in XDF:units. * BSCALE is part of XDF:units. * BZERO is XDF:units/@offset * BLANK is replaced by XDF:array/@noDataValue. * CTYPEn is replaced by XDF:axis/name. * GROUPS, PCOUNT, GCOUNT, AND END are not needed in XML. The element parameter is used for random parameters and well-formedness takes care of proper grouping and ending.. * PTYPE is replaced by XDF:parameter/name. * EXTEND, XTENSION, EXTLEVEL are not needed in XDF which accepts any number of arrays in a hierarchical structure. * EXTNAME is replaced by XDF:array/name. * TFIELDS is no longer needed. * TBCOL is not needed. The first column of a row is numbered 0. * TFORM info is held in XDF:dataFormat. * TTYPEn is XDF:field/name, TUNIT is XDF:field/units. * TSCALn is held in XDF:field/units. * TZEROn has not been considered. * TNULLn seems like XDF:field/@noDataValue.