fix wrong assert in notify_ready_to_unpack

The offset will always be >=4 because we store the CellStatus in the
data sent to p4est in front of the user data. This means the smallest
offset will 4, no matter how much data is attached.
add test
add documentation to register_data_attach
Merge pull request #51 from tjhei/fix_data_attach

fix, test, document Triangulation::register_data_attach

 * fix a bug in an assertion
 * add a test
 * document how the CellStatus in the callback function works