make an AstroDBHeader class and methods

[kelle]

From @dr-rodriguez :

class AstroDBHeader(Header):
     def __init__(self, header=None):  
          # prepare empty header if none is provided
     def add_missing_keywords(self, format, keywords):  
          # add missing keywords
     # etc etc

what that allows you to do is essentially add extra methods to your original header so you can do things like

header = AstroDBHeader()
header.add_missing_keywords()
header.check_header()

and everything else defined in fits.py