Implement WriteTemplateHeader action

[kabasset]

The action would add some specified keywords from a user-provided list, e.g. TELESCOP. Other keywords could be interpreted, e.g. DATE.

Here is a tentative list of keywords:

• ORIGIN
• AUTHOR
• REFERENC
• TELESCOP
• INSTRUME
• DATE-OBS
• OBSERVER
• DATE (computable)
• DATAMINMAX (computable)
• TDMIN/MAXn (computable)