[fitsbits] Abuse of EXTEND keyword

[Phil Hodge]

> What will be a file with no extension in your convention ?
>
>  - no keyword at all
>  - EXTEND=F and no NEXTEND kwd
>  - EXTEND=F and NEXTEND=0
>  - EXTEND=T and NEXTEND=0


For HST data we write NEXTEND in files that are supposed to have 
extensions, and in that case it's useful in that it tells a calibration 
program how many exposures (for example) to expect.  A utility program 
might not even look for NEXTEND.

A program that expects data in extensions would typically use a default 
value if the NEXTEND keyword were absent.  If NEXTEND = 0, there's no 
data to process.  EXTEND is almost always ignored.

Phil