TODO

o Sketch out process & at some point split it out from ndmapper.
o Use NDData .mask instead of .flags?
  - Is NDDataArray still needed?
o Add Python meta-decorator for iterating over files, copying ref headers etc.?
o Make sure lazy loading is not triggered when instantiating NDLater from
  another instance.
o Should NDMapIO keep a reference to its data so they can be shared between
  NDLater instances whether or not they have already been loaded?
o Add save() & reload() methods to DataFileList.  

Later

o IRAF tests with additional files (inc. VAR/DQ).
o Override other list methods properly in DataFileList.
  - Should it inherit from ABCs instead?
o User interface to Table instances & organization of Table idents.
o Add calibration association persistence.
o Complete IRAF logging behaviour.
o Add auto .fits (and other known types) in DataFile?
o Capture stdout/stderr?
  - Sometimes warnings go here that should end up in the log.
  - How to deal with stdout/log duplication & asynchronous status output?