To Do

- Add ability for units to be extracted from item comments

- Read HISTORY from NDF and merge it into FITS header

- 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)

- Tied interface should be modified so that key_COMMENT will retrieve
  the comment associated with key "key" rather than the value (this will
  not clash with real headers because the 8 character limit will be
  exceeded)

- Support MEF fits headers in a single object. Can use a special card
  type that will indicate a header change [possible with the name
  of the next subfile]. Could use "1_" prefix to reference the
  items from sub headers.
