The GFitsHeader class now derives from GContainer and implements all generic container class methods (at(), size(), isempty(), append(), insert(), remove(), reserve(), extend()). The hascard() methods have been renamed to contains() for compliance with other container classes. The card() methods have been renamed to at(), also for compliance. The card access operators and methods now return the cards by reference. Card access operators[] have been implemented to access header cards without range checking.
The cards are now stored using a std::vector structure.
