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.