[fitsbits] programming interfaces for WCS defaults

[David Berry]

Steve,

> Note that it is not helpful to assert that at least one of the WCS
> keywords for a particular version must exist in order to instantiate
> that version.  The author of a FITS HDU may simply insert
> WCSNAMEA= 'My silly instance of the default world coordinate system'
> along with no other keywords for WCS version A, and by so doing the
> author has instantiated WCS version A with a WCS that is not really
> useful.

Presumably, if the author has decided to include a value for WCSNAMEA
then the author at least must consider the associated default aleternate
axis descriptions to be useful. Maybe it's simply a case of trusting the
author to decide what is useful and what isn't.

David