Add write() and writeFile().
Put several box types into ATOM_WITH_CHILDREN.
Merge AtomWithChildren into Atom.
Finish Atom.write().

* Add some more boxes into FULL_BOX.
* Add read24().
* Remove handling of largesize and 0-size from parse_atom().
They are handled in Atom's constructor now.
* Add version, flags, largesize and uuids, the optionals in header.
* Add Atom.get_actual_size().
Merge branch 'feature/separate_header_and_data' into develop

Atom.write() is done.