Primary header

The file will contain a header and a number of BINTABLE extensions. The main header will include the following information :

1. MODLNAME - the name of the model (string of 12 characters max).
2. MODLUNIT - the units for the model (string of 12 characters max).
3. REDSHIFT - whether redshift is to be a parameter (logical).
4. ADDMODEL - whether this is an additive model (logical).

It should also include the standard OGIP keywords :

HDUCLASS 		 'OGIP'           


HDUCLAS1 		 'XSPEC TABLE MODEL'    


HDUVERS 		 '1.0.0'