disable "Save XTF" button when no channels selected
minor source code formatting
"Save SEG-Y" button (placeholder for now)
move [un]wrap to `sacker` module
support 'x' padding format in sacker.py
sacker.wrap doctest and module docstring
put \x00 back to doctests (use raw doc literal)
sacker.wrap: assume 0 for missing `data` keys
sacker: allow comments in spec
Sacker class (anologous to struct.Struct); set endian
nicer regexp for parsing wrap specs
xtf.py: dict_factory => data_factory
SEG-Y write