To Do
-----

- Add ability for units to be extracted from item comments

- Read HISTORY from NDF and merge it into FITS header. On writing the
  merged HISTORY should be writing to the HISTORY extension of the NDF
  as this is regarded as canonical.

- Provide option to extract WCS from FITS header and store it in AST object

- Support subheaders (ie those in Multi-extension fits files that are
  not part of the main header, or the headers in UKIRT data (.I1, .I2).
  
  As of Version 2.6 this is partially complete, you can have an Item of type
  HEADER which simply points to another Astro::FITS::Header object, and appears
  as hashes when using a tied object.
  
  However there is no code to generate those subheaders automatically when
  reading an MEF file.

