Write CDi_j even when zero

[PaulPrice]

Our users have requested that CDi_j be written even if they're zero. The FITS WCS standard says that these keywords are interpreted as zero if missing, but makes no statement about whether they should be present or not if zero.

Can we add an attribute controlling whether CDi_j will be written if they're zero? I'm happy to do the work if this is approved.

[PaulPrice]

To avoid proliferation of attributes in AST, I'll achieve this via post-processing the result from AST.